hash of mpz_t and mpq_t

[Hans Åberg]

> On 16 Jun 2021, at 15:38, Hans Åberg <haberg-1 at telia.com> wrote:
> 
> Otherwise using _mp_d ^ (mp_limb_t)_mp_size might do.

That is ^ std::hash<int>()(_mp_size) on the hash value gotten from _mp_d. The resulting hash value is size_t in C++.