"Free" cheatsheet

This was posted in the Haskell Slack:
> you could model the free "whatever" [i.e. Free Monad, Free Applicative, etc.] generated by a given functor as:  
> ```purescript
> -- Using poly kinds' kind signatures here for clarity of idea
> type Free :: ((Type -> Type) -> Constraint) -> (Type -> Type) -> Type -> Type
> type Free c f a = forall g. c g => (f ~> g) -> g a
> ```