Lost MonadFix instances

[박신환]

instance MonadFix Down where
mfix f = Down (fix (\x -> let
Down x' = f x
in x'
))

instance MonadFix Proxy where
mfix _ = Proxy

instance MonadFix U1 where
mfix _ = U1

instance MonadFix Complex where
mfix f = fix (realPart . f) :+ fix (imagPart . f)