Support underscores for mpz/mpq assignments from strings

[Sergey B Kirpichev]

Hello,

a common feature of modern languages is using underscores as visual
separators for digit grouping purposes in integral, floating-point and
complex number literals, see:
https://www.python.org/dev/peps/pep-0515/

Some projects add workarounds for the gmp to allow this syntax, e.g.:
https://github.com/ocaml/Zarith/pull/75

I think, the gmp can accept this syntax.  Tentative patch attached.
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