Re-add state monad (or provide tips for why it's not needed in clojure)

[caioaao]

I've seen some cases where a state monad was the only way to write proper functional code in clojure. If removing state monad is the final decision, it'd be cool if the docs would include an explanation on why it's not needed in clojure, with some examples :)

[niwinz]

If this is really usefull, I'm open to a PR for reverting that change ;)

[sovelten]

I'm working with a modified version of the state monad implementation of the previous cats version, I can open a PR for that with the changes I made, the previous implementation had some minor issues that I fixed

[sovelten]

#210