Proposal: unify constant functors

[Simon Hengel]

> +1 for going Brent and Ross's way, (removing Const from Control.Applicative
> and moving Data.Functor.Constant into base)

I'd prefer if Control.Applicative.Const would only be deprecated (for
now).

    {-# DEPRECATED Const "use Data.Functor.Constant.Constant instead" #-}

Otherwise, +1.