Hash plain finite-field vectors and matrices

[fingolfin]

Add a generic finite-field vector hash helper that computes the
minimal field of the entries via BaseDomain and feeds
NumberFFVector. Reuse it for non-compressed matrices by hashing
rows with the existing matrix-list combiner while leaving
compressed GF(2) and 8bit methods alone.

Resolves #77

Co-authored-by: Codex codex@openai.com

[codecov]

Codecov Report

❌ Patch coverage is 82.35294% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.50%. Comparing base (2298578) to head (bed438e).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
gap/hash.gi	81.81%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #78      +/-   ##
==========================================
+ Coverage   52.41%   52.50%   +0.09%     
==========================================
  Files          16       16              
  Lines        5869     5891      +22     
==========================================
+ Hits         3076     3093      +17     
- Misses       2793     2798       +5     

Files with missing lines	Coverage Δ
gap/hash.gd	100.00% <100.00%> (ø)
gap/hash.gi	55.66% <81.81%> (+0.70%)	⬆️

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