gmp 0.3.1

The following functions are intentionally left out of the bindings:

• gmp_randinit (not thread-safe, obsolete)
• mpz_random (not thread-safe, obsolete)
• mpz_random2 (not thread-safe, obsolete)
• mpf_set_default_prec (not thread-safe)
• mpf_get_default_prec (not thread-safe)
• mpf_init (not thread-safe)
• mpf_inits (not thread-safe, va_list wrapper)
• mpf_clears (va_list wrapper)
• mpf_swap (no better than rust's swap)
• mpf_set_prec_raw (could be exposed with an unsafe function if needed)
• mpz_inits (va_list wrapper)
• mpz_clears (va_list wrapper)
• mpz_swap (no better than rust's swap)
• mpq_inits (va_list wrapper)
• mpq_clears (va_list wrapper)
• mpq_swap (no better than rust's swap)