feat(persistent): More efficient HugrView iterators for PersistentHugr

[lmondada]

This PR improves persistent-hugr performance. It does three things

1. Introduces PersistentHugrCache to cache the list of children of each commit.
2. Improves the implementation of HugrView::children by adding a custom iterator, instead of materialising the entire hugr in memory.
3. small refactor: move fn node_status and enum NodeStatus from hugr-persistent/src/wire.rs to hugr-persistent/src/persistent_hugr.rs

Note:

Beyond the first two children, the order of

persistent_hugr.children(node)

and

let hugr = persistent_hugr.to_hugr();
hugr.children(node);

may no longer match when node is a Dataflow parent. This is annoying but I could not find an easy way to fix this. AFAIK this should not lead to observable differences according to current specs, but might be an issue at some point. I've created issue #2618 to track this.

[codecov]

Codecov Report

❌ Patch coverage is 95.37037% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.39%. Comparing base (3e12b65) to head (b9f183a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
hugr-persistent/src/trait_impls/utils.rs	93.33%	2 Missing ⚠️
hugr-persistent/src/persistent_hugr.rs	95.65%	0 Missing and 1 partial ⚠️
hugr-persistent/src/trait_impls.rs	97.72%	0 Missing and 1 partial ⚠️
hugr-persistent/src/wire.rs	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2595      +/-   ##
==========================================
+ Coverage   83.36%   83.39%   +0.02%     
==========================================
  Files         257      259       +2     
  Lines       50664    50741      +77     
  Branches    46187    46264      +77     
==========================================
+ Hits        42238    42313      +75     
- Misses       6059     6061       +2     
  Partials     2367     2367              

Flag	Coverage Δ
python	91.53% <ø> (ø)
rust	82.60% <95.37%> (+0.02%)	⬆️

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[aborgna-q]

Nice!

It's a bit annoying that we need to force evaluation of the whole children when popping stuff from the back, but there's no way around it.

[aborgna-q]

Add some docs!

[lmondada]

yup will do!

[aborgna-q]

We only need to wrap the IterValidNodes case, Either::Right(children) is already a doubleEndedIterator

[lmondada]

true, fixing that!

[lmondada]

It's a bit annoying that we need to force evaluation of the whole children when popping stuff from the back, but there's no way around it.

True, but tbh it's barely (ever?) used backwards.