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.

That first 0 is the "count" parameter which which no-longer give the user access to, so the count calculation was coming out at 1 and exporting the "\0" string. The test failing was being masked by trying to emulate what count would have done with  a substr on the result. I've fixed this now.