Factorial2 (!!) of an integer is an integer

`factorial2` of an integer is now known to be an integer. This is done via an implementation of `_eval_is_integer`.
it's got to be `fuzzy_and((self.args[0].is_integer, (self.args[0] + 1).is_nonnegative))` because if either is None, the answer should be None, not False, right?