Remove backtrace (#1464)

### What
Moved backtrace in the Host's DebugInfo behind a new feature.

### Why
The backtrace output is included in the Debug formatting of errors when
testutils are enabled, but the trace has not turned out to be that
useful in contract tests written with the SDK.

When an error occurs in a contract test the output includes two long
stack traces. The first is the one provided by backtrace. The second is
one provided by the Rust test harness. Both are very similar and contain
approximately the same information. The Rust test harness' trace is
actually a little easier to read because it's not quite so verbose.

Here's a before and after of what a test failing looks like before and
after this change:

#### Before

```
HostError: Error(Auth, InvalidAction)

Event log (newest first):
   0: [Diagnostic Event] topics:[error, Error(Auth, InvalidAction)], data:"escalating error to panic"
   1: [Diagnostic Event] topics:[error, Error(Auth, InvalidAction)], data:["contract call failed", exec, [CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAFCT4]]
   2: [Failed Diagnostic Event (not emitted)] contract:CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAHK3M, topics:[error, Error(Auth, InvalidAction)], data:"caught error from function"
   3: [Failed Diagnostic Event (not emitted)] contract:CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAHK3M, topics:[error, Error(Auth, InvalidAction)], data:"escalating error to panic"
   4: [Failed Diagnostic Event (not emitted)] contract:CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAHK3M, topics:[error, Error(Auth, InvalidAction)], data:["Unauthorized function call for address", CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAFCT4]
   5: [Diagnostic Event] topics:[fn_call, Bytes(0000000000000000000000000000000000000000000000000000000000000003), exec], data:CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAFCT4
   6: [Diagnostic Event] topics:[fn_call, Bytes(0000000000000000000000000000000000000000000000000000000000000005), __constructor], data:Void
   7: [Diagnostic Event] topics:[fn_call, Bytes(0000000000000000000000000000000000000000000000000000000000000004), __constructor], data:Void
   8: [Diagnostic Event] topics:[log], data:["holder_2", CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAK3IM]
   9: [Diagnostic Event] topics:[log], data:["holder_1", CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAITA4]
   10: [Diagnostic Event] topics:[log], data:["test_id", CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAHK3M]
   11: [Diagnostic Event] topics:[fn_call, Bytes(0000000000000000000000000000000000000000000000000000000000000003), __constructor], data:Void
   12: [Diagnostic Event] topics:[log], data:["auth_id", CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAFCT4]
   13: [Diagnostic Event] contract:CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAFCT4, topics:[fn_return, init], data:Void
   14: [Diagnostic Event] topics:[fn_call, Bytes(0000000000000000000000000000000000000000000000000000000000000002), init], data:[CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAD2KM, 2]
   15: [Diagnostic Event] topics:[fn_call, Bytes(0000000000000000000000000000000000000000000000000000000000000002), __constructor], data:Void
   16: [Diagnostic Event] topics:[fn_call, Bytes(0000000000000000000000000000000000000000000000000000000000000001), __constructor], data:Void

Backtrace (newest first):
   0: backtrace::backtrace::libunwind::trace
             at /Users/leighmcculloch/.cargo/registry/src/index.crates.io-6f17d22bba15001f/backtrace-0.3.74/src/backtrace/libunwind.rs:116:5
      backtrace::backtrace::trace_unsynchronized
             at /Users/leighmcculloch/.cargo/registry/src/index.crates.io-6f17d22bba15001f/backtrace-0.3.74/src/backtrace/mod.rs:66:5
   1: backtrace::backtrace::trace
             at /Users/leighmcculloch/.cargo/registry/src/index.crates.io-6f17d22bba15001f/backtrace-0.3.74/src/backtrace/mod.rs:53:14
   2: backtrace::capture::Backtrace::create
             at /Users/leighmcculloch/.cargo/registry/src/index.crates.io-6f17d22bba15001f/backtrace-0.3.74/src/capture.rs:292:9
   3: backtrace::capture::Backtrace::new_unresolved
             at /Users/leighmcculloch/.cargo/registry/src/index.crates.io-6f17d22bba15001f/backtrace-0.3.74/src/capture.rs:287:9
   4: soroban_env_host::host::error::<impl soroban_env_host::host::Host>::maybe_get_debug_info::{{closure}}
             at /Users/leighmcculloch/.cargo/git/checkouts/rs-soroban-env-230c62c6b1f3d3f1/75b7821/soroban-env-host/src/host/error.rs:299:37
   5: soroban_env_host::budget::Budget::with_shadow_mode
             at /Users/leighmcculloch/.cargo/git/checkouts/rs-soroban-env-230c62c6b1f3d3f1/75b7821/soroban-env-host/src/budget.rs:1039:21
   6: soroban_env_host::host::Host::with_debug_mode
             at /Users/leighmcculloch/.cargo/git/checkouts/rs-soroban-env-230c62c6b1f3d3f1/75b7821/soroban-env-host/src/host.rs:650:24
   7: soroban_env_host::host::error::<impl soroban_env_host::host::Host>::maybe_get_debug_info
             at /Users/leighmcculloch/.cargo/git/checkouts/rs-soroban-env-230c62c6b1f3d3f1/75b7821/soroban-env-host/src/host/error.rs:296:13
   8: soroban_env_host::host::error::<impl soroban_env_host::host::Host>::error::{{closure}}
             at /Users/leighmcculloch/.cargo/git/checkouts/rs-soroban-env-230c62c6b1f3d3f1/75b7821/soroban-env-host/src/host/error.rs:280:23
   9: soroban_env_host::budget::Budget::with_shadow_mode
             at /Users/leighmcculloch/.cargo/git/checkouts/rs-soroban-env-230c62c6b1f3d3f1/75b7821/soroban-env-host/src/budget.rs:1039:21
  10: soroban_env_host::host::Host::with_debug_mode
             at /Users/leighmcculloch/.cargo/git/checkouts/rs-soroban-env-230c62c6b1f3d3f1/75b7821/soroban-env-host/src/host.rs:650:24
  11: soroban_env_host::host::error::<impl soroban_env_host::host::Host>::error
             at /Users/leighmcculloch/.cargo/git/checkouts/rs-soroban-env-230c62c6b1f3d3f1/75b7821/soroban-env-host/src/host/error.rs:267:9
  12: <soroban_env_host::host::Host as soroban_env_common::env::EnvBase>::escalate_error_to_panic
             at /Users/leighmcculloch/.cargo/git/checkouts/rs-soroban-env-230c62c6b1f3d3f1/75b7821/soroban-env-host/src/host.rs:838:26
  13: soroban_sdk::env::internal::reject_err::{{closure}}
             at /Users/leighmcculloch/.cargo/git/checkouts/rs-soroban-sdk-0ab58a716c671239/a101940/soroban-sdk/src/env.rs:52:23
  14: core::result::Result<T,E>::map_err
             at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/core/src/result.rs:854:27
  15: soroban_sdk::env::internal::reject_err
             at /Users/leighmcculloch/.cargo/git/checkouts/rs-soroban-sdk-0ab58a716c671239/a101940/soroban-sdk/src/env.rs:52:9
  16: <soroban_sdk::env::Env as soroban_env_common::env::Env>::call
             at /Users/leighmcculloch/.cargo/git/checkouts/rs-soroban-sdk-0ab58a716c671239/a101940/soroban-sdk/src/env.rs:1796:13
  17: soroban_sdk::env::Env::invoke_contract
             at /Users/leighmcculloch/.cargo/git/checkouts/rs-soroban-sdk-0ab58a716c671239/a101940/soroban-sdk/src/env.rs:379:18
  18: auth::tests::TestContractClient::exec
             at /Users/leighmcculloch/Code/talk-stellar-meridian24-testing/auth/src/tests.rs:17:1
  19: auth::tests::unit_test
             at /Users/leighmcculloch/Code/talk-stellar-meridian24-testing/auth/src/tests.rs:61:5
  20: auth::tests::unit_test::{{closure}}
             at /Users/leighmcculloch/Code/talk-stellar-meridian24-testing/auth/src/tests.rs:35:15
  21: core::ops::function::FnOnce::call_once
             at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/core/src/ops/function.rs:250:5


stack backtrace:
   0: rust_begin_unwind
             at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/std/src/panicking.rs:665:5
   1: core::panicking::panic_fmt
             at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/core/src/panicking.rs:74:14
   2: <soroban_env_host::host::Host as soroban_env_common::env::EnvBase>::escalate_error_to_panic
             at /Users/leighmcculloch/.cargo/git/checkouts/rs-soroban-env-230c62c6b1f3d3f1/75b7821/soroban-env-host/src/host.rs:839:9
   3: soroban_sdk::env::internal::reject_err::{{closure}}
             at /Users/leighmcculloch/.cargo/git/checkouts/rs-soroban-sdk-0ab58a716c671239/a101940/soroban-sdk/src/env.rs:52:23
   4: core::result::Result<T,E>::map_err
             at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/core/src/result.rs:854:27
   5: soroban_sdk::env::internal::reject_err
             at /Users/leighmcculloch/.cargo/git/checkouts/rs-soroban-sdk-0ab58a716c671239/a101940/soroban-sdk/src/env.rs:52:9
   6: <soroban_sdk::env::Env as soroban_env_common::env::Env>::call
             at /Users/leighmcculloch/.cargo/git/checkouts/rs-soroban-sdk-0ab58a716c671239/a101940/soroban-sdk/src/env.rs:1796:13
   7: soroban_sdk::env::Env::invoke_contract
             at /Users/leighmcculloch/.cargo/git/checkouts/rs-soroban-sdk-0ab58a716c671239/a101940/soroban-sdk/src/env.rs:379:18
   8: auth::tests::TestContractClient::exec
             at ./src/tests.rs:17:1
   9: auth::tests::unit_test
             at ./src/tests.rs:61:5
  10: auth::tests::unit_test::{{closure}}
             at ./src/tests.rs:35:15
  11: core::ops::function::FnOnce::call_once
             at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/core/src/ops/function.rs:250:5
  12: core::ops::function::FnOnce::call_once
             at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/core/src/ops/function.rs:250:5
```

#### After

```
HostError: Error(Auth, InvalidAction)

Event log (newest first):
   0: [Diagnostic Event] topics:[error, Error(Auth, InvalidAction)], data:"escalating error to panic"
   1: [Diagnostic Event] topics:[error, Error(Auth, InvalidAction)], data:["contract call failed", exec, [CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAFCT4]]
   2: [Failed Diagnostic Event (not emitted)] contract:CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAHK3M, topics:[error, Error(Auth, InvalidAction)], data:"caught error from function"
   3: [Failed Diagnostic Event (not emitted)] contract:CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAHK3M, topics:[error, Error(Auth, InvalidAction)], data:"escalating error to panic"
   4: [Failed Diagnostic Event (not emitted)] contract:CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAHK3M, topics:[error, Error(Auth, InvalidAction)], data:["Unauthorized function call for address", CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAFCT4]
   5: [Diagnostic Event] topics:[fn_call, Bytes(0000000000000000000000000000000000000000000000000000000000000003), exec], data:CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAFCT4
   6: [Diagnostic Event] topics:[fn_call, Bytes(0000000000000000000000000000000000000000000000000000000000000005), __constructor], data:Void
   7: [Diagnostic Event] topics:[fn_call, Bytes(0000000000000000000000000000000000000000000000000000000000000004), __constructor], data:Void
   8: [Diagnostic Event] topics:[log], data:["holder_2", CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAK3IM]
   9: [Diagnostic Event] topics:[log], data:["holder_1", CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAITA4]
   10: [Diagnostic Event] topics:[log], data:["test_id", CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAHK3M]
   11: [Diagnostic Event] topics:[fn_call, Bytes(0000000000000000000000000000000000000000000000000000000000000003), __constructor], data:Void
   12: [Diagnostic Event] topics:[log], data:["auth_id", CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAFCT4]
   13: [Diagnostic Event] contract:CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAFCT4, topics:[fn_return, init], data:Void
   14: [Diagnostic Event] topics:[fn_call, Bytes(0000000000000000000000000000000000000000000000000000000000000002), init], data:[CAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAD2KM, 2]
   15: [Diagnostic Event] topics:[fn_call, Bytes(0000000000000000000000000000000000000000000000000000000000000002), __constructor], data:Void
   16: [Diagnostic Event] topics:[fn_call, Bytes(0000000000000000000000000000000000000000000000000000000000000001), __constructor], data:Void

stack backtrace:
   0: rust_begin_unwind
             at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/std/src/panicking.rs:665:5
   1: core::panicking::panic_fmt
             at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/core/src/panicking.rs:74:14
   2: <soroban_env_host::host::Host as soroban_env_common::env::EnvBase>::escalate_error_to_panic
             at /Users/leighmcculloch/Code/rs-soroban-env/soroban-env-host/src/host.rs:839:9
   3: soroban_sdk::env::internal::reject_err::{{closure}}
             at /Users/leighmcculloch/.cargo/git/checkouts/rs-soroban-sdk-0ab58a716c671239/a101940/soroban-sdk/src/env.rs:52:23
   4: core::result::Result<T,E>::map_err
             at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/core/src/result.rs:854:27
   5: soroban_sdk::env::internal::reject_err
             at /Users/leighmcculloch/.cargo/git/checkouts/rs-soroban-sdk-0ab58a716c671239/a101940/soroban-sdk/src/env.rs:52:9
   6: <soroban_sdk::env::Env as soroban_env_common::env::Env>::call
             at /Users/leighmcculloch/.cargo/git/checkouts/rs-soroban-sdk-0ab58a716c671239/a101940/soroban-sdk/src/env.rs:1796:13
   7: soroban_sdk::env::Env::invoke_contract
             at /Users/leighmcculloch/.cargo/git/checkouts/rs-soroban-sdk-0ab58a716c671239/a101940/soroban-sdk/src/env.rs:379:18
   8: auth::tests::TestContractClient::exec
             at ./src/tests.rs:17:1
   9: auth::tests::unit_test
             at ./src/tests.rs:61:5
  10: auth::tests::unit_test::{{closure}}
             at ./src/tests.rs:35:15
  11: core::ops::function::FnOnce::call_once
             at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/core/src/ops/function.rs:250:5
  12: core::ops::function::FnOnce::call_once
             at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/core/src/ops/function.rs:250:5
```

### Note

I'm not attached to this change if we need back traces for something,
but if we need to keep them, I'm interested in how we can silence them
for errors in contract tests.

### Bonuses

As a bonus, this removes 7 deps from the dep graph for contract
developers! 🎉

soroban-env-host/Cargo.toml

@@ -105,7 +105,8 @@ default-features = false
 features = ["arbitrary"]
 
 [features]
-testutils = ["soroban-env-common/testutils", "recording_mode", "dep:backtrace"]
+testutils = ["soroban-env-common/testutils", "recording_mode"]
+backtrace = ["dep:backtrace"]
 next = ["soroban-env-common/next", "stellar-xdr/next"]
 tracy = ["dep:tracy-client", "soroban-env-common/tracy"]
 recording_mode = []

soroban-env-host/src/host/error.rs

@@ -5,7 +5,7 @@ use crate::{
     ConversionError, EnvBase, Error, Host, TryFromVal, U32Val, Val,
 };
 
-#[cfg(any(test, feature = "testutils"))]
+#[cfg(any(test, feature = "backtrace"))]
 use backtrace::{Backtrace, BacktraceFrame};
 use core::fmt::Debug;
 use std::{
@@ -18,7 +18,7 @@ use super::metered_clone::MeteredClone;
 #[derive(Clone)]
 pub(crate) struct DebugInfo {
     events: Events,
-    #[cfg(any(test, feature = "testutils"))]
+    #[cfg(any(test, feature = "backtrace"))]
     backtrace: Backtrace,
 }
 
@@ -67,12 +67,12 @@ impl DebugInfo {
         Ok(())
     }
 
-    #[cfg(not(any(test, feature = "testutils")))]
+    #[cfg(not(any(test, feature = "backtrace")))]
     fn write_backtrace(&self, _f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         Ok(())
     }
 
-    #[cfg(any(test, feature = "testutils"))]
+    #[cfg(any(test, feature = "backtrace"))]
     fn write_backtrace(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         // We do a little trimming here, skipping the first two frames (which
         // are always into, from, and one or more Host::err_foo calls) and all
@@ -296,8 +296,11 @@ impl Host {
             self.with_debug_mode(|| {
                 if let Ok(events_ref) = self.0.events.try_borrow() {
                     let events = events_ref.externalize(self)?;
-                    let backtrace = Backtrace::new_unresolved();
-                    res = Some(Box::new(DebugInfo { backtrace, events }));
+                    res = Some(Box::new(DebugInfo {
+                        #[cfg(any(test, feature = "backtrace"))]
+                        backtrace: Backtrace::new_unresolved(),
+                        events,
+                    }));
                 }
                 Ok(())
             });