Mark push_next() as unsafe and add push_next_one() alternative

`push_next()` is totally `unsafe` because of dereferencing a chain of
`p_next` pointers to find the end of the chain to insert, which was
obfuscated by a large `unsafe` block for the `BaseOutStructure` pointer
cast in commit c8c8f69 ("`next` can contain a pointer chain and we need
to correct insert it.").

While this function should definitely be marked unsafe, wrapping
builders in `unsafe {}` en masse in user code isn't all too desirable,
especially when this soundness issue only exists to optionally
walk a `p_next` chain while most users are likely inserting bare
structs without pointer chains most of the time.  `push_next_one()`
is introduced for this reason, remaining safe to call without any
unintended raw pointer dereferences.

README.md

@@ -91,19 +91,20 @@ let device: Device = instance
 
 ### Pointer chains
 
-Use `base.push_next(ext)` to insert `ext` at the front of the pointer chain attached to `base`.
+Use `base.push_next_one(ext)` to insert `ext` at the front of the pointer chain attached to `base`.  If `ext` already contains a pointer chain of its own,
+call `unsafe` `push_next()` instead.
 
 ```rust
 let mut variable_pointers = vk::PhysicalDeviceVariablePointerFeatures::default();
 let mut corner = vk::PhysicalDeviceCornerSampledImageFeaturesNV::default();
 
 let mut device_create_info = vk::DeviceCreateInfo::default()
-    .push_next(&mut corner)
-    .push_next(&mut variable_pointers);
+    .push_next_one(&mut corner)
+    .push_next_one(&mut variable_pointers);
 ```
 
-The generic argument of `.push_next()` only allows valid structs to extend a given struct (known as [`structextends` in the Vulkan registry](https://registry.khronos.org/vulkan/specs/1.3/styleguide.html#extensions-interactions), mapped to `Extends*` traits).
-Only structs that are listed one or more times in any `structextends` will implement a `.push_next()`.
+The generic argument of `.push_next_one()` only allows valid structs to extend a given struct (known as [`structextends` in the Vulkan registry](https://registry.khronos.org/vulkan/specs/1.3/styleguide.html#extensions-interactions), mapped to `Extends*` traits).
+Only structs that are listed one or more times in any `structextends` will implement a `.push_next_one()`.
 
 ### Flags and constants as associated constants
 

ash/src/lib.rs

@@ -204,8 +204,8 @@ mod tests {
             <*mut _>::cast(&mut corner),
         ];
         let mut device_create_info = vk::DeviceCreateInfo::default()
-            .push_next(&mut corner)
-            .push_next(&mut variable_pointers);
+            .push_next_one(&mut corner)
+            .push_next_one(&mut variable_pointers);
         let chain2: Vec<*mut vk::BaseOutStructure<'_>> = unsafe {
             vk::ptr_chain_iter(&mut device_create_info)
                 .skip(1)