ext/gmp: Fix GMP operator RHS overflow for GMP values

[LamentXU123]

<?php
$too_large = gmp_init("18446744073709551616");

var_dump(gmp_init(2) ** $too_large);
var_dump(gmp_init(2) << $too_large);

object(GMP)#3 (1) {
  ["num"]=>
  string(1) "1"
}
object(GMP)#2 (1) {
  ["num"]=>
  string(1) "2"
}

This PR fix the overflow by rejecting large gmp objects in these calculation (throws ValueError)

[iliaal]

LGTM. One optional thing: the new test could add a >> case since it goes through the same helper.

[LamentXU123]

Thank you!