Add num trait implementations to U256 and I256

[sigurd4]

I'm developing a crate for customizable floating point types and need your U256 type to make a 256-bit octuple float. However my code relies on various traits from the crate num. I've implemented these for both U256 and I256 as an optional feature. It would be useful to have the traits implemented for it, either from this crate, or from the num-traits and num-integer crates. This way, everything is standardized and generic. I hope my contribution is welcome.