Add methods in Decimal to allow interaction with PHP's native numbers

[castarco]

In order to allow a more straightforward and efficient interaction with Decimal objects, it would be a great step if we add specific methods to avoid the need of creating new Decimal objects on the fly every time we have to operate with PHP's native numbers.

The current methods (add, mul, div, and so on) have hinting, in order to allow interpreter/compiler optimizations. I propose to create equivalents with the prefix n, for example: mul 's native equivalent will be nMul .