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.

Might it be better to throw an error in the data_len%size!=0 case instead?

In any case, I don't get why count gets incremented here - wouldn't that lead to reads past the data boundary?