Add fq_default_ctx_init_modulus and fq_default_is_invertible.
Add fq_default_ctx_init_modulus and fq_default_is_invertible.

wbhart/flint2,wbhart/flint2,wbhart/flint2,fredrik-johansson/flint2,fredrik-johansson/flint2,fredrik-johansson/flint2