feat: Draft for hugr-model with export, import, parsing and pretty printing

[zrho]

This PR defines a draft of the model format, and the following parts that operate on the format:

• Import: A module in the model format can be imported as a core hugr module.
• Export: A core hugr module can be exported into the model format.
• Parsing: A model hugr can be parsed from an s-expression based text format.
• Pretty printing:: A model hugr can be printed into the s-expression format, with readable indentation.

This PR is very big, and yet it does not present a completed feature. The model and core format differ in several aspects. Where they differ, this is mostly intentional: The model is kept flexible enough to incorporate features that we know that we want in the future but are currently hard or impossible to express in the core. Import and export perform conversions where possible, but do not yet cover everything.

To prevent this PR from becoming even bigger and to allow for core and model to converge incrementally from both directions, I suggest the following plan: We merge this PR into main with the model related code being feature gated and considered experimental. We then perform tweaks to model and core in smaller PRs where necessary until they are sufficiently close.

[codecov]

Codecov Report

Attention: Patch coverage is 69.09570% with 704 lines in your changes missing coverage. Please review.

Project coverage is 85.71%. Comparing base (fc3efe5) to head (5c9299f).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
hugr-core/src/import.rs	70.15%	122 Missing and 109 partials ⚠️
hugr-model/src/v0/text/print.rs	59.32%	116 Missing and 76 partials ⚠️
hugr-model/src/v0/text/parse.rs	65.89%	70 Missing and 92 partials ⚠️
hugr-core/src/export.rs	82.55%	67 Missing and 19 partials ⚠️
hugr-model/src/v0/mod.rs	42.10%	33 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1542      +/-   ##
==========================================
- Coverage   87.43%   85.71%   -1.72%     
==========================================
  Files         127      132       +5     
  Lines       21740    24017    +2277     
  Branches    18740    21017    +2277     
==========================================
+ Hits        19008    20587    +1579     
- Misses       1964     2366     +402     
- Partials      768     1064     +296     

Flag	Coverage Δ
python	92.46% <ø> (ø)
rust	84.75% <69.09%> (-1.88%)	⬇️

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[ss2165]

first review of mod.rs

[aborgna-q]

Some general comments, I'll do mod/export next.

Other tidbits:

• Add hugr-model to .github/change-filters.yml

[cqc-alec]

(Looked at parse.rs and print.rs.) LGTM, I just highlighted some TODO comments that you may or may not want to address here (if not we should make issues).

[ss2165]

import first pass, second half coming soon

[ss2165]

import round 2

[zrho]

Given that the code is deliberately declared unstable and after looking over what the tests cover, I am currently fine with the extent of test coverage. This will increase as more features are built around it. So if you agree do not let that be an obstacle to approving.

[cqc-alec]

Happy to approve the parts I looked at.

[ss2165]

Happy to approve mod and import

[aborgna-q]

Same here. Re-request the review once there's no more expected changes.