Support underscores for mpz/mpq assignments from strings

[Hans Åberg]

> On 11 Jun 2021, at 15:02, Hans Åberg <haberg-1 at telia.com> wrote:
> 
> So to facilitate that, you might have a special function that indicates which characters should be discarded, and the decimal separators. The international standard mentioned above would require the latter to be a string, like ",.". Setting the former to " " would be the C/C++ behavior, "" would not discard anything.

It is of course rare for integers and rational numbers to have decimal separators :-), and it may not be worth the effort to add it for the floating point numbers.