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.

I think multiple endianness options (and also multiple order options) should results in a warning and failure return value. Why is the `case 0` case in there?