feat: Represent order edges in hugr-model as metadata.

[zrho]

This PR introduces metadata to encode order edges in hugr-model. The children of a dataflow region can be assigned a key via order_hint.key metadata. Then order_hint.order metadata on the dataflow region encodes order edges between keys. The PR includes the necessary import and export code in hugr-core.

[codecov]

Codecov Report

Attention: Patch coverage is 57.35736% with 142 lines in your changes missing coverage. Please review.

Project coverage is 82.92%. Comparing base (5dd1f96) to head (e13e8d5).

Files with missing lines	Patch %	Lines
hugr-model/src/capnp/hugr_v0_capnp.rs	24.39%	61 Missing and 1 partial ⚠️
hugr-core/src/import.rs	52.00%	8 Missing and 16 partials ⚠️
hugr-py/src/hugr/model/export.py	8.00%	23 Missing ⚠️
hugr-model/src/v0/ast/python.rs	0.00%	11 Missing ⚠️
hugr-py/src/hugr/envelope.py	30.00%	7 Missing ⚠️
hugr-py/src/hugr/model/__init__.py	66.66%	4 Missing ⚠️
hugr-core/src/envelope.rs	25.00%	0 Missing and 3 partials ⚠️
hugr-model/src/v0/binary/read.rs	80.00%	0 Missing and 3 partials ⚠️
hugr-core/src/package.rs	95.00%	0 Missing and 1 partial ⚠️
hugr-model/src/v0/ast/mod.rs	90.90%	0 Missing and 1 partial ⚠️
... and 3 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2027      +/-   ##
==========================================
- Coverage   83.09%   82.92%   -0.18%     
==========================================
  Files         215      215              
  Lines       40895    41167     +272     
  Branches    37109    37345     +236     
==========================================
+ Hits        33981    34137     +156     
- Misses       5030     5132     +102     
- Partials     1884     1898      +14     

Flag	Coverage Δ
python	85.32% <30.00%> (-0.53%)	⬇️
rust	82.67% <62.19%> (-0.14%)	⬇️

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[codspeed-hq]

CodSpeed Performance Report

Merging #2027 will not alter performance

_{Comparing zrho/model-order-hints (e13e8d5) with main (5dd1f96)}

Summary

✅ 21 untouched benchmarks

[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/v0.40.0/src/lints/enum_variant_added.ron

Failed in:
variant ImportError:OrderHint in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-core/src/import.rs:72

--- failure function_missing: pub fn removed or renamed ---

Description:
A publicly-visible function cannot be imported by its prior path. A `pub use` may have been removed, or the function itself 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/v0.40.0/src/lints/function_missing.ron

Failed in:
function hugr_core::import::import_hugr, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/import.rs:83
function hugr_core::export::export_hugr, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-core/src/export.rs:31