Fix binary polynomial division

[angeris]

Summary

• implement polynomial degree calculation
• fix divrem for binary polynomials
• test polynomial division corner cases
• use consistent type variable name in tests

Testing

• julia --project=BinaryFields -e 'using Pkg; Pkg.test()' (fails: julia not found)
• attempted curl -fsSL https://install.julialang.org | bash -s -- -y (failed: couldn't connect to server)

[angeris]

again, having fun with codex; need to actually test this and bench it to make sure there isn't a performance regression, but we only use this function for the (rare) inverse, which should be precomputed anyways