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.

Why is empty data_len disallowed here? Isn't that how a 0 value is going to be exported?

Also, I don't understand this error message ... I'd drop the magic and just say that the input length must be a multiple of the size parameter.