Add the ability to derive instances of Functor, Foldable, Traversable

Add the ability to derive instances of Functor, Foldable, Traversable

This patch is a straightforward extension of the 'deriving' mechanism.
The ability to derive classes Functor, Foldable, Traverable is controlled
by a single flag  -XDeriveFunctor.  (Maybe that's a poor name.)

Still to come: documentation

Thanks to twanvl for developing the patch