LambdaCase \cases syntax

[mixphix]

Currently we have support for \case highlighting, but \cases (provided by this recently-merged GHC proposal) is not yet supported:

foo :: Maybe a -> Maybe b -> Maybe (a, b)
foo = \case
  Nothing -> Nothing
  Just a -> \case
    Nothing -> Nothing
    Just b -> Just (a, b)

bar :: Maybe a -> Maybe b -> Maybe (a, b)
bar = \cases
  (Just a) (Just b) -> Just (a, b)
  _ _ -> Nothing

I have already prepared a fork with a patch!