Add Backwards

[treeowl]

The indexed equivalent of Control.Applicative.Backwards.Backwards:

newtype IxBackwards m i j a = IxBackwards {ixForwards :: m j i a}

instance IxFunctor f => IxFunctor (IxBackwards f) where
  imap f (IxBackwards x) = IxBackwards (imap f x)

instance IxPointed f => IxPointed (IxBackwards f) where
  ireturn = IxBackwards . ireturn

instance IxApplicative f => IxApplicative (IxBackwards f) where
  iap (IxBackwards fs) (IxBackwards xs) =
    IxBackwards $ imap (flip ($)) xs `iap` fs