feat: Constant values in hugr-model

[zrho]

Import and export constant values via hugr-model.

Includes terms corresponding to Value::Function and allows exporting; importing these is left for a later PR since sum/extension constants are the most important for now. In the future, we should move from the JSON/typetag based model for extension constants to one based on custom constructors.

[hugrbot]

This PR contains breaking changes to the public Rust API.
Please deprecate the old API instead (if possible), or mark the PR with a ! to indicate a breaking change.

cargo-semver-checks summary

--- failure enum_variant_added: enum variant added on exhaustive enum ---

Description:
A publicly-visible enum without #[non_exhaustive] has a new variant.
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#enum-variant-new
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.38.0/src/lints/enum_variant_added.ron

Failed in:
variant Term:Const in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-model/src/v0/mod.rs:571
variant Term:ConstFunc in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-model/src/v0/mod.rs:677
variant Term:ConstAdt in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-model/src/v0/mod.rs:683
variant Operation:Const in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-model/src/v0/mod.rs:380

--- failure enum_variant_missing: pub enum variant removed or renamed ---

Description:
A publicly-visible enum has at least one variant that is no longer available under its prior name. It may have been renamed or removed entirely.
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.38.0/src/lints/enum_variant_missing.ron

Failed in:
variant Term::Quote, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-model/src/v0/mod.rs:565

[codecov]

Codecov Report

Attention: Patch coverage is 65.97938% with 99 lines in your changes missing coverage. Please review.

Project coverage is 86.55%. Comparing base (09cbc6a) to head (030e601).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
hugr-core/src/import.rs	54.25%	17 Missing and 26 partials ⚠️
hugr-core/src/export.rs	85.71%	15 Missing and 2 partials ⚠️
hugr-model/src/v0/text/print.rs	47.61%	8 Missing and 3 partials ⚠️
hugr-model/src/v0/binary/read.rs	41.17%	9 Missing and 1 partial ⚠️
hugr-model/src/v0/text/parse.rs	52.38%	4 Missing and 6 partials ⚠️
hugr-model/src/v0/binary/write.rs	42.85%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1838      +/-   ##
==========================================
- Coverage   86.67%   86.55%   -0.13%     
==========================================
  Files         194      194              
  Lines       35010    35156     +146     
  Branches    31823    31969     +146     
==========================================
+ Hits        30346    30430      +84     
- Misses       2939     2968      +29     
- Partials     1725     1758      +33     

Flag	Coverage Δ
python	92.31% <ø> (ø)
rust	85.98% <65.97%> (-0.14%)	⬇️

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[ss2165]

Looks good, biggest question is about -> placement

[ss2165]

if these are meant to stay in sync would be better to have a dedicated struct holding both that has responsibility for their correctness.

[zrho]

Ideally yes. I'm not entirely sure yet if the way that functions are exported is ideal here, so there might be a bit of churn around these things happening. In that light I'm not sure how much it is worth to polish.

[ss2165]

leave a todo perhaps

[zrho]

TODO added.

[ss2165]

LGTM modulo CI fixes 👍