Generalize Functor and Traversable for FreeT

[mniip]

I have just been bitten by the Monad constraints on these instances...

It is worth noting that I believe it's impossible to write an Applicative m => Applicative (FreeT f m) instance that would be in agreement with ap, so Applicative and Alternative are sadly tainted with monad.

[mniip]

Just realized how this affects pre-AMP haskell. Should I rather add Functor constraints on subclasses, or only enable the generalized instances on new ghcs?

[mniip]

Done the former because it made more sense. Would it be possible to, perhaps, do something along the lines of:

import Prelude hiding (Monad)
import qualified Prelude (Monad)

type Monad m = (Functor m, Monad m)

as a pre-AMP compatibility layer? Just an idea.

[fizruk]

Can/should we also generalise Traversable for FT?
It should be possible, although current implementation depends on Monad m constraint apparently.

[RyanGlScott]

Superseded by #225.