simplify the distribute Return case

This commit shouldn't change the meaning of the code:

    lift (Return r) =
     -- by Monad instance of Stream
    lift (return r) =
     -- by laws of MonadTrans/lift
    return r 

However, the result may be more efficient. In the case where t = Stream f, we have:

    lift (Return r) =
     -- by definition of lift
    (Effect . fmap Return) (Return r) =
     -- by definition of (.)
    Effect (fmap Return (Return r)) =
     -- by definition of fmap
    Effect (Return (Return r))

But doing `return r` directly produces just `Return r` (without the no-op Effect wrapper).

Author: mauke

src/Streaming/Internal.hs

@@ -763,7 +763,7 @@ distribute :: (Monad m, Functor f, MonadTrans t, MFunctor t, Monad (t (Stream f
            => Stream f (t m) r -> t (Stream f m) r
 distribute = loop where
   loop stream = case stream of
-    Return r     -> lift (Return r)
+    Return r     -> return r
     Effect tmstr -> hoist lift tmstr >>= loop
     Step fstr    -> join (lift (Step (fmap (Return . loop) fstr)))
 {-# INLINABLE distribute #-}