GMP function to generate prime number bigger than X

[Hans Petter Selasky]

Hi,

I didn't find any functions to quickly generate a prime number bigger 
than X in GMP, so I made my own for now.

I have a special SAT solving application, which need arbitrarily sized 
prime number, from 8-bits to several thousand bits and even more.

The algorithm is inside hprsat_set_global_modulus() at:

https://github.com/hselasky/libhpsat/blob/main/hpRsat_prime.cpp

It basically consists of generating smaller prime numbers to quickly 
build up a bigger one.

--HPS