gmp_import() and gmp_export()

These functions are thin wrappers around `mpz_import()` and `mpz_export()` provided by the GMP library.

They provide fast conversion between large GMP numbers and binary strings, with configurable word order, word size, and endianness.

> Exporting a GMP num with a value of 0 correctly exports as "\0"

Are you sure about that? Your export tests contain three lines like `['0',0,1,GMP_BIG_ENDIAN,'']` and the mpz_export docs always say this should result in an empty string.