feat: Bytes literal in hugr-model.

[zrho]

Adds a term for byte literals to hugr-model.

In the text format, byte literals are represented as base64 encoded strings.

Closes #1545.

[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:Bytes in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-model/src/v0/mod.rs:691
variant Term:BytesType in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-model/src/v0/mod.rs:697

[codecov]

Codecov Report

Attention: Patch coverage is 81.08108% with 7 lines in your changes missing coverage. Please review.

Project coverage is 86.55%. Comparing base (774a13e) to head (6972f60).

Files with missing lines	Patch %	Lines
hugr-core/src/import.rs	0.00%	3 Missing ⚠️
hugr-model/src/v0/text/parse.rs	70.00%	1 Missing and 2 partials ⚠️
hugr-model/src/v0/binary/read.rs	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1845      +/-   ##
==========================================
- Coverage   86.55%   86.55%   -0.01%     
==========================================
  Files         194      194              
  Lines       35156    35193      +37     
  Branches    31969    32006      +37     
==========================================
+ Hits        30430    30462      +32     
- Misses       2968     2970       +2     
- Partials     1758     1761       +3     

Flag	Coverage Δ
python	92.31% <ø> (ø)
rust	85.98% <81.08%> (+<0.01%)	⬆️

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