Miscellaneous changes

[fingolfin]

• Remove dead code
• Merge Set+List into just Set
• Remove redundant MutableCopyMat calls
• Replace One(GL(n,F)) by One(A)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.53%. Comparing base (09ff465) to head (bad1f11).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #60      +/-   ##
==========================================
- Coverage   95.54%   95.53%   -0.01%     
==========================================
  Files           7        7              
  Lines        1009     1008       -1     
==========================================
- Hits          964      963       -1     
  Misses         45       45              

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