Fixes #8488

(This is my first PR :D )
Implemented _eval_is_prime for the Mul class in core module

Example:
k = Symbol('k', integer=True)
(9*k).is_prime
False
Ohh, sorry, I didn't knew that. Can you give me a hint on how can I determine if something like this "Mul(4, S.Half, evaluate=False)" is prime? The only way I think is to evaluate it first (and I can't use evalf either right?)