lib/math: Added gcd method on Int.

Signed-off-by: Lucas Bajolet <r4pass@hotmail.com>
Merge: lib/math: Added gcd method on Int.

Added GCD method using Stein's algorithm on Int.

Pull-Request: #490
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>