Add more setcoeff! methods and use them to deduplicate PolyRingElem constructors

[lgoettgens]

Unfortunately this won't work as I try to access attributes/parents of elements in setcoeff! before they are set.
I see two ways to resolve this issue:

1. All element constructors should get a parent. This will be a bigger change, in particular if we want to have this consistent for all poly types.
2. setcoeff! gets an optional fourth argument, that is a UInt for nmod_poly-based types and a fmpz_mod_ctx_struct for fmpz_mod_poly-based types. If this fourth argument is not passed, we can reconstruct it (in many cases, but not inside of the constructors as parents are not available).

I would tend to 2 for now, and maybe remove this fourth argument again once 1 has made enough progress. What do you think @fingolfin ?