Optimizing mul method of point class with BcMath

Now the change is correct. The fail in the earlier pull request was mine. I had trouble dealing with the site and ended up doing wrong. Sorry.

I'm very interested in using ECDH on a website, but the server where it will be hosted does not have GMP, only BCMath. I really liked your code and I'm trying to optimize it when using the bcmath. The replacement I made in the Mul method of the Point class managed save about 33% in some operations, how to generate public key or secret key. The original code was repeating the execution of bcmath_Utils::bcand.