Make liftF a method of MonadFree

[treeowl]

This seems to work a bit better for things like FT.

[treeowl]

This isn't strictly backwards compatible because MonadFree f m now requires that either:

1. f is a Functor, or
2. f does not need to be a Functor to implement liftF.

These hold for all the implementations in the package. I don't know if they fail somewhere else.

[treeowl]

Hmm... That's a bit weird... the instance for Control.Monad.Free.Free actually didn't previously need a Functor f constraint, but it had one anyway.

[treeowl]

Another option is to use a different name for the method, to preserve performance characteristics.