Use nmod_poly_minimal_irreducible for qadic context creation with small p

[alexey-orlov-math]

When no Conway polynomial is available and the characteristic fits in a ulong, use nmod_poly_minimal_irreducible instead of the fmpz_mod_poly_randtest_sparse_irreducible. The former is a deterministic search with heavy pruning, improving creation time significantly.

The net result is, locally, from

julia> @time qadic_field(2,1024;precision=1)
  3.849074 seconds (481.74 k allocations: 3.087 GiB, 0.99% gc time, 0.06% compilation time)
(Unramified extension of 2-adic numbers of degree 1024, (2^0 + O(2^1))*a)

julia> @time qadic_field(2,2048;precision=1)
 53.298277 seconds (4.44 M allocations: 37.756 GiB, 2.27% gc time, 0.01% compilation time)
(Unramified extension of 2-adic numbers of degree 2048, (2^0 + O(2^1))*a)

to creating qadic contexts for [2, 3, 5, 7, 11]^[123, 628, 1024, 2048, 2755, 2867] in 15.9 seconds

[fredrik-johansson]

Thanks!