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'm wondering whether we might want to drop the `nails` parameter. GMP docs indicate that this is targeted at some extremely rare system (vector cray system) which uses weird integer representations. Doesn't seem relevant to PHP.