Generalize MonadAccum w (AccumT w m).

There is no reason to restrict this instance any further.

Author: jumper149

Control/Monad/Accum.hs

@@ -160,7 +160,7 @@ class (Monoid w, Monad m) => MonadAccum w m | m -> w where
   {-# MINIMAL accum | look, add #-}
 
 -- | @since 2.3
-instance (Monoid w) => MonadAccum w (AccumT w Identity) where
+instance (Monoid w, Monad m) => MonadAccum w (AccumT w m) where
   look = Accum.look
   add = Accum.add
   accum = Accum.accum