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.

case 0 for when only GMP_ORDER_* is specified in the options, the default value is assigned. Although, I could just not assign anything and it keeps the default given to it in the function... makes more sense.