feat: separate DeadCodeElimPass, fix constant-folding of Modules

[acl-cqc]

• Constant Folding now just breaks wires and inserts Const + LoadConst even for "unnecessary" (DCE-able) ops
• Followup DCE pass removes these
• DCE pass allows explicit specification of entry points for Module-rooted Hugrs
  • constant folding assumes main entry-point (in common with dataflow analysis, for now) - I've added a test for constant folding: FuncDefns and FuncDecls are not considered needed #1797 that this fixes
• DCE pass takes an arbitrary callback for NodeDivergence aka "purity" - naming suggestions most welcome here

Could use more DCE-specific tests.

Thus, two in one:
closes #1797, #1807

[codecov]

Codecov Report

Attention: Patch coverage is 83.21678% with 24 lines in your changes missing coverage. Please review.

Project coverage is 86.52%. Comparing base (d6b8681) to head (595e82e).

Files with missing lines	Patch %	Lines
hugr-passes/src/dead_code.rs	80.00%	16 Missing ⚠️
hugr-passes/src/const_fold/test.rs	79.31%	0 Missing and 6 partials ⚠️
hugr-passes/src/const_fold.rs	95.45%	0 Missing and 1 partial ⚠️
hugr-passes/src/lib.rs	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1897      +/-   ##
==========================================
- Coverage   86.56%   86.52%   -0.05%     
==========================================
  Files         195      197       +2     
  Lines       35724    35794      +70     
  Branches    32537    32607      +70     
==========================================
+ Hits        30925    30971      +46     
- Misses       3012     3030      +18     
- Partials     1787     1793       +6     

Flag	Coverage Δ
python	92.34% <ø> (ø)
rust	85.95% <83.21%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[acl-cqc]

Merging with #1896