hand a Go pointer to C code but must separately arrange for
Go to hang on to a reference to the pointer until C is done with it.
*/
package gmp

/*
#cgo LDFLAGS: -lgmp
#include <gmp.h>
#include <stdlib.h>

// gmp 5.0.0+ changed the type of the 3rd argument to mp_bitcnt_t,
// so, to support older versions, we wrap these two functions.
void _mpz_mul_2exp(mpz_ptr a, mpz_ptr b, unsigned long n) {