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.

Wasn't sure about this. If you specify `GMP_ENDIAN_LITTLE | GMP_ENDIAN_BIG` which is obviously impossible, you will end up with `GMP_ENDIAN_NATIVE` - In effect, bad options will leave you with the default. 

Perhaps better to
```
	case GMP_ENDIAN_NATIVE:
	case 0:
		*endian = 0;
		break;
	default:
		php_error_docref(NULL TSRMLS_CC, E_WARNING, ...
		*endian = 0;
```

Thoughts?