Add fmpq_poly_discriminant

[fredrik-johansson]

As observed in flintlib/python-flint#312 (comment), there is no fmpq_poly_discriminant.

[oscarbenjamin]

I suppose fmpq_poly_discriminant is fairly trivial given that it can use fmpz_poly_discriminant or fmpq_poly_resultant.

These methods are also missing for fq_default_poly. Maybe they could use the gr_poly_resultant implementation?

Looks like gr_poly_discriminant is also missing.

[fredrik-johansson]

The only difficulty with implementing _gr_poly_discriminant in terms of _gr_poly_resultant is that dividing by the leading coefficient can fail in a non-integral domain.

In that case, instead of calling _gr_poly_resultant directly, one could use a modification of _gr_poly_resultant_sylvester which symbolically omit this factor from the first column before computing the determinant.