Avoid redundant computations in finite field .conjugate() method

[remyoudompheng]

📚 Description

The previous implementation was factoring the order (characteristic^degree) to obtain the field degree.
The methods cardinality() and order() are really defined as return self.characteristic()**self.degree().

Sage 10rc3

sage: p = next_prime(2**160)
sage: K = GF((p,2))
sage: x = K.random_element()
sage: %timeit x.conjugate()
13.2 ms ± 77.8 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

After patch:

sage: %timeit x.conjugate()
5.01 µs ± 15.5 ns per loop (mean ± std. dev. of 7 runs, 100,000 loops each)

📝 Checklist

• The title is concise, informative, and self-explanatory.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation accordingly.

⌛ Dependencies

None

[github-actions]

Documentation preview for this PR is ready! 🎉
Built with commit: 8d09044

[yyyyx4]

Looks good, thank you!