Remove hacks that allowed passing GF(q) instead of GF(q^2)

[fingolfin]

These hacks were needed back when we did not pass around the fields explicitly. But they always were a crutch: they help us to deal with the case where e.g. q = p or q = p^3 ("so the user already took the square root")

Some tests relied on the removed workaround, so adjust those as well.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.47%. Comparing base (1375b8b) to head (dae4073).
⚠️ Report is 15 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #137      +/-   ##
==========================================
- Coverage   94.51%   94.47%   -0.04%     
==========================================
  Files          14       14              
  Lines        4684     4669      -15     
==========================================
- Hits         4427     4411      -16     
- Misses        257      258       +1     

Files with missing lines	Coverage Δ
gap/Forms.gi	87.27% <100.00%> (-0.84%)	⬇️

... and 1 file with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.