WIP: Added InstanceSigs to Contravariant, Foldable, Bifoldable, Bitraversable and...

Me tidying up, let me know if you'd prefer a different way of contributing. I added instance signatures to some example declarations:

-- > instance Bitraversable Either where
-- >   bitraverse :: Applicative f => (a -> f c) -> (b -> f d) -> (Either a b -> f (Either c d))
-- >   bitraverse f _ (Left x)  = Left <$> f x
-- >   bitraverse _ g (Right y) = Right <$> g y
-- >
-- > instance Bitraversable (,) where
-- >   bitraverse :: Applicative f => (a -> f c) -> (b -> f d) -> ((a, b) -> f (c, d))
-- >   bitraverse f g (x, y) = (,) <$> f x <*> g y

I would really like to add instance signatures to every method, has this discussion happened? Instance signatures are valuable like top-level signatures.