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`.
test search for this pattern in the sympy/core is [here](https://gist.githubusercontent.com/skirpichev/03fb41f52bcc6d3653df/raw/06c8df41ccc0200e2091c9cdfb7c48682416233a/a.log)