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 picked least significant words first because this is a selfish development, and it better fits my own needs. If you think MSW first is really more sane, I'm happy to change it.

What do you think about adding constants for these values? I don't think users should be expected to remember what all of the -1/0/1 flags mean.