feat: Introduce new enum for constant folding; deprecate Value::Function

[acl-cqc]

We've been using ops::Value for constant-folding, which is a bit of a shortcut but leads to significant problems: the result of LoadFunction cannot fit into a Value (you'd need to copy all other external functions used by the loaded one inside it, or something - not really practical, and you've lost the understanding that you were calling another function in the same Hugr, too). #2059 solved this inside dataflow analysis, but this extends the approach to constant folding, by allowing to feeding "function pointers" (from LoadFunction) into constant-folding.

The "solution" of extending Value to allow a reference to a node in the containing Hugr was considered in #1856 and was roundly rejected. Instead, this PR adds a separate enum FoldVal that looks quite similar to Value but adds those references/function-pointers.

I've taken the liberty of not including nested Hugrs in FoldVal, but rather deprecating Value::Function in favour of getting front-ends to lift these into their own FuncDefn's in the Hugr. I could be persuaded not to, and add a nested-Hugr variant to FoldVal, if we really want, but it does seem like probably more effort than it's worth - does anyone really have a good use case for Value::Function ?

I've also deprecated the old constant_fold and fold routines in favour of the new ones, which are currently called fold2. Better naming suggestions are welcome....(an alternative might be to define a new version of the ConstantFolder trait with only the new method, make OpDef store one of those, deprecate the old trait, and then impl<T:ConstantFolder> NewConstantFolder for T ?)

NOTE/TODO: implementing the deprecated fn fold does not trigger a deprecation warning (!), only calling it

Also note: this is not intended to be part of the v0.16.0 release; it can be a non-breaking followup, but I'm targetting the release branch just because this depends on things in that release.

closes: #2087, #1856

[codecov]

Codecov Report

Attention: Patch coverage is 87.26115% with 20 lines in your changes missing coverage. Please review.

Project coverage is 83.28%. Comparing base (d8a5d67) to head (e147cbe).

Files with missing lines	Patch %	Lines
hugr-passes/src/const_fold/value_handle.rs	56.00%	11 Missing ⚠️
hugr-core/src/extension/const_fold.rs	94.52%	3 Missing and 1 partial ⚠️
hugr-core/src/extension/op_def.rs	75.00%	3 Missing ⚠️
hugr-core/src/ops/custom.rs	75.00%	1 Missing ⚠️
hugr-passes/src/const_fold.rs	97.14%	1 Missing ⚠️

Additional details and impacted files

@@                  Coverage Diff                   @@
##           release-rs-v0.16.0    #2060      +/-   ##
======================================================
- Coverage               83.33%   83.28%   -0.06%     
======================================================
  Files                     219      218       -1     
  Lines                   42205    42223      +18     
  Branches                38307    38428     +121     
======================================================
- Hits                    35173    35164       -9     
- Misses                   5221     5248      +27     
  Partials                 1811     1811              

Flag	Coverage Δ
python	85.42% <ø> (-0.31%)	⬇️
rust	83.06% <87.26%> (-0.03%)	⬇️

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