Merge branch 'ticket/16910' into HEAD
Fix mpz_t, mpq_t, mpf_t declarations
Use mpz_srcptr for padics pow_computer
Trac #15946: Cython warnings about mpz_t ("... referenced before assignment")

Similar to #16583, we should fix the declarations for `mpz_t` (and
related types). This also involves fixing:
* various bad usages of `mpz_t` (it was `ctypedef`'ed as `void*`,
meaning you could do with it what you wanted and Cython wouldn't
complain)
* changing many functions to pass an `mpz_t` instead of an `mpz_t*`. For
example the NTL interface functions `mpz_to_ZZ` and `ZZ_to_mpz` and the
`pow_computer` functions from p-adics.

See also [https://groups.google.com/forum/#!msg/sage-
devel/bs4lnNwbINs/B2iVgxK7pCYJ]

URL: http://trac.sagemath.org/15946
Reported by: mmezzarobba
Ticket author(s): Jeroen Demeyer
Reviewer(s): Volker Braun