Add gr_derivative, with implementations for polynomial rings

[rburing]

Comments welcome about the fallback behavior, and the need for another variant that takes a gr_ptr for the variable.

Derivatives are not yet implemented for fmpz_mpoly_q, fmpz_mod_mpoly_q, or symbolic expressions, so I did not cover them here. Also fmpq_mpoly does not have a GR context yet, so it is not covered either.

P.S. I struggled a bit where to put the respective lines in each file. Right now it's mostly after powering. I can change this if someone has a proposal where it should go instead.

[fredrik-johansson]

I think I would prefer gr_derivative to take a gr_srcptr for the variable, the function you've implemented here being called gr_derivative_gen or something like that. Maybe others have an opinion?

P.S. I struggled a bit where to put the respective lines in each file. Right now it's mostly after powering. I can change this if someone has a proposal where it should go instead.

It's never going to be completely consistent, so best trying not thinking too much about that kind of thing :-)