New code for primality testing

[paul zimmermann]

> And it could be specified that this function
>   int mpz_millerrabin (mpz_t n, int reps)
> internally takes care of randstate boilerplate, like in current code.
> Which adds to its added value and simplicity of use.
> Repeated calls may not bring additional confidence,
> but it's not important because the number of "calls" can already be set with the reps parameter.

in some contexts, we want to get a full primality test for moderate size
numbers. For that purpose, it would be more useful to provide a list of
(user chosen) bases to mpz_millerrabin, or at least that the bases chosen
by a given 'reps' argument are documented.

See https://en.wikipedia.org/wiki/Miller%E2%80%93Rabin_primality_test.

Paul