Add hoist_invariants() directive for semiring reductions

[alexreinking]

This PR adds a scheduling directive similar to rfactor that hoists invariant factors according to a detected distributive (semiring) law into the write-back part of the accumulation step. This enables writing straightforward quantized kernels in the algorithm language and factoring them into efficient kernels.

This is meant to work towards optimizing quantized mat-mul and mat-vec kernels (especially those from GGML).

Checklist

• Tests added or updated (not required for docs, CI config, or typo fixes)
• Documentation updated (if public API changed)
• Python bindings updated (if public API changed)
• Benchmarks are included here if the change is intended to affect performance.
• Commits include AI attribution where applicable (see Code of Conduct)

[codecov]

Codecov Report

❌ Patch coverage is 84.86842% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.14%. Comparing base (4620569) to head (e588356).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Func.cpp	84.86%	8 Missing and 15 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9190      +/-   ##
==========================================
+ Coverage   70.02%   70.14%   +0.12%     
==========================================
  Files         258      259       +1     
  Lines       78702    79272     +570     
  Branches    19164    19344     +180     
==========================================
+ Hits        55110    55609     +499     
- Misses      17884    17899      +15     
- Partials     5708     5764      +56     

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[abadams]

Sadly need to delete the separable downsample test, because it doesn't do what we hoped, but LGTM for the rest of it.