Introduce structs corresponding 1-1 to FLINT structs, use them inside our wrappers like e.g. `FpFieldElem` or `fqPolyRepFieldElem`

[fingolfin]

          (...) We could (... add ...) a separate struct `fq_nmod_t` (and `fmpz_t` etc. etc.) which faithfully replicate the FLINT structs, nothing more and nothing less. Then e.g. `fqPolyRepFieldElem` could start with a member `x:: fq_nmod_t`. 

That way, we could resolve all the lies in the code everywhere. But that would be a major piece of work for relatively limited gain (if I were to start Nemo today, that's more or less what I'd do anyway, but as it is now...).

Originally posted by @fingolfin in #1871 (comment)

[thofma]

For some types there are already _struct versions, e.g. qqbar_struct.

Claus and I discussed at various points rewriting the wrapper code, also to deal with some of the current shortcomings. Just saying this so so that we don't duplicate efforts.