Which nf_elem functions support delayed reduction?

[fredrik-johansson]

It's not clear which nf_elem functions are meant to support delayed reduction. For example, nf_elem_add does, but nf_elem_add_fmpq and nf_elem_scalar_mul_fmpq (surprisingly) do not.

It would be great if this was documented, and consistently tested.

In fact, nf_elem_mul_red is not tested at all, which is a bit worrying.