Atomic Traits

[zommiommy]

Hi! I've added my implementation of Atomic and IntoAtomic, I use them really often and they are quite similar to your traits so I think that they might be a good fit in your library. I still have to write documentation and examples, but I wanted to know what you think about this.

Also, we could add the opt-in support for the methods from the unstable feature #![feature(atomic_from_mut)] by adding an enabling feature to the create.