Fixed limb-amount gmp_int

Will fixed-limb amount ```gmp_int``` be welcomed? Why? To generalize the fixed-precision number concept (and for some of my own usage).

Such an implementation will not be a straight ```mpz_t``` wrapper, but it will be built around GMP using the same concept, which is being used for fixed-precision ```cpp_int```.

Start by reviewing the existing fixed-precision cpp_int concept and the GMP-backed implementation approach described in the issue. Before coding, clarify whether a fixed-limb gmp_int is wanted, its intended behavior, and what acceptance criteria would define a complete implementation.

Written by the indexing model from the issue text.