support larger int types

[paul zimmermann]

       Dear Marc,

> I could have used them years ago...

me too. I also vote for mpz_[gs]et_uint64 and mpz_[gs]et_int64:

void mpz_set_uint64 (mpz_t z, uint64_t q);
void mpz_set_int64 (mpz_t z, int64_t q);
uint64_t mpz_get_uint64 (mpz_srcptr z);
int64_t mpz_get_int64 (mpz_srcptr z);

This would make life easier on 32-bit plaforms when we want to deal with
64-types. We had to write such functions in CADO-NFS since GMP doesn't
provide them.

Best regards,
Paul