Set up the hack to allow better overloads of the Mul trait.

This is strongly inpired by
http://smallcultfollowing.com/babysteps/blog/2012/10/04/refining-traits-slash-impls/
"Replaced" Scalar{Add, Sub, Mul, Div} by operator overloading.

Those traits are not really removed since rust cannot handle those multiple operator overloading
very well yet, making them sometimes unuseable on generic code.