Arbitrary-precision numbers

The gmp-mpfr crate has been replaced by rug. The main features of the rug crate are:

• big integers with arbitrary precision,
• big rational numbers with arbitrary precision,
• multi-precision floating-point numbers with correct rounding, and
• multi-precision complex numbers with correct rounding.

The crate is free software: you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. See the full text of the GNU LGPL and GNU GPL for details.