Made a derive macro to easily get all of an enum's variants

I just learned the basics of macros and figured I'd give a shot trying to solve a problem I've had for a while. Theres just one derive trait in this crate, Variants, that when derived will generate a constant array that holds all of the enum's variants along with a method that exposes a static reference to the constant array.

Give it a look, leave some feedback, maybe even open up a PR. I hope you like what you see!