Include mini-mpq

[Marc Nieper-Wißkirchen]

 Hi,

This is a request to integrate the missing parts of the mini-gmp library
into Gnulib.  To be more specific, mini-mpq is currently missing in Gnulib;
see [1].

Thanks,

Marc

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[1] https://gmplib.org/repo/gmp/file/tip/mini-gmp