Support underscores for mpz/mpq assignments from strings

[Marc Glisse]

On Thu, 10 Jun 2021, Sergey B Kirpichev wrote:

> 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.

There is nothing special about the underscore, English uses ',', French 
uses '.', some use a space, C++ seems to go for "'", etc.

C locales have thousands_sep (and grouping), similar to decimal_point, 
although I don't think we support it, and it only seems allowed before the 
decimal point, strangely enough.

If we add support for separators, we need to do it in a way that allows 
the user the specify the separator, either through the locale or through 
an argument.

I don't know if mpz_inp_str is the right function to introduce this, I 
would expect to see it in gmp_*scanf first, but others would know better.

-- 
Marc Glisse