Add side-channel attack resilient `powModSecInteger`

This is a follow-up to 97c101b7363f84d925a600acb56a9fa3a997ea0d which
introduced the "ordinary" `powModInteger` operation.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
Expose two GMP primality operations

This exposes `mpz_probab_prime_p()` and `mpz_nextprime()` as
`testPrimeInteger` and `nextPrimeInteger` respectively and is especially
useful for cryptographic algorithms such as RSA.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>