Warn when a function's type indicates that it's unnecessarily monadic

Motivation

Newbies to Haskell often write functions like f x y = return (x + y + 10), which drag monads in unnecessarily.

Proposal

When a function uses monads unnecessarily, emit a warning. I think this definition of "unnecessarily" is both correct and rigorous enough to implement: if a function has a polymorphic type, a type variable appears only in positive position, and no constraints refer to that type variable except for Functor, Applicative, Monad, and MonadFix instances, then it's unnecessarily monadic.