Moved 'Kleisli' from 'Arrow' to 'Monad' module.

That's where it belongs, and now the 'Arrow' class can readily be used in monad utility-functions.
Add arrow infix combinators.
Add '<$>' to 'Functor' module.
Changed signature of 'pure', now completely equivalent to 'return' again.

The previous 'a -> f b' signature didn't make sense for such method, as the 'Category' instance could not be deduced from anything.
Add 'Traversable' class, at the moment in 'Monad' module.

(Where it totally doesn't belong!)