Actually get rid of strictness on left monadic fold accumulator

Author: andrewthad

containers/src/Data/Map/Internal.hs

@@ -3371,7 +3371,7 @@ foldlWithKeyM f z = go z
   where
     go z' Tip             = return z'
     go z' (Bin _ kx x l r) = do
-      !z'' <- go z' l
+      z'' <- go z' l
       z''' <- f z'' kx x
       go z''' r
 {-# INLINE foldlWithKeyM #-}