ext/gmp: Add gmp_prevprime()

[LamentXU123]

Implement #13661

This is built on top of #16613

[ndossche]

I think we should throw when the previous prime does not exist rather than return a GMP 0 number. 0 isn't a prime after all...

[TimWolla]

Also according to GMP’s documentation (https://gmplib.org/manual/Number-Theoretic-Functions#index-mpz_005fprevprime):

Return 1 if rop is a probably prime, and 2 if rop is definitely prime.

These functions use a probabilistic algorithm to identify primes. For practical purposes it’s adequate, the chance of a composite passing will be extremely small.

[LamentXU123]

mathematically it makes no sense to output a prime number less than 2. I'd suggest to directly throw ValueError in these cases in pure PHP spirits.