feat: Improved representation for metadata in hugr-model

[zrho]

Closes #1848 by using a term-based representation for metadata items.

[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:Meta in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-model/src/v0/mod.rs:720

--- failure struct_missing: pub struct removed or renamed ---

Description:
A publicly-visible struct cannot be imported by its prior path. A `pub use` may have been removed, or the struct 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/v0.38.0/src/lints/struct_missing.ron

Failed in:
struct hugr_model::v0::MetaItem, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-model/src/v0/mod.rs:506

[codecov]

Codecov Report

Attention: Patch coverage is 71.05263% with 11 lines in your changes missing coverage. Please review.

Project coverage is 86.52%. Comparing base (87cb536) to head (06e0f36).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
hugr-core/src/import.rs	66.66%	1 Missing and 2 partials ⚠️
hugr-model/src/v0/binary/write.rs	40.00%	3 Missing ⚠️
hugr-model/src/v0/text/print.rs	60.00%	2 Missing ⚠️
hugr-core/src/export.rs	92.30%	1 Missing ⚠️
hugr-model/src/v0/binary/read.rs	66.66%	1 Missing ⚠️
hugr-model/src/v0/text/parse.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1849      +/-   ##
==========================================
- Coverage   86.54%   86.52%   -0.02%     
==========================================
  Files         194      194              
  Lines       35264    35250      -14     
  Branches    32077    32063      -14     
==========================================
- Hits        30518    30500      -18     
- Misses       2969     2974       +5     
+ Partials     1777     1776       -1     

Flag	Coverage Δ
python	92.31% <ø> (ø)
rust	85.94% <71.05%> (-0.02%)	⬇️

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[croyzor]

LGTM