Two small fixes

1. Restore a `Data.Semigroup.Commutative` reexport accidentally removed
   in #56. (I accidentally forgot to notice during review.)

2. Replace partial `Map.lookup` with proper custom error for internal
   error. (This would make debugging a bug in the implementation
   easier.) This was part of #55.

Co-Authoredy-By: Dmitry Olshansky <olshanskydr@gmail.com>

src/Data/Patch.hs

@@ -28,6 +28,7 @@ import Control.Applicative (liftA2)
 import Data.Semigroup (Semigroup (..))
 #endif
 
+import qualified Data.Semigroup.Commutative as X
 import Data.Patch.Class as X
 import Data.Patch.DMap as X hiding (getDeletions)
 import Data.Patch.DMapWithMove as X

src/Data/Patch/MapWithPatchingMove.hs

@@ -231,8 +231,9 @@ patchThatSortsMapWith cmp m = PatchMapWithPatchingMove $ Map.fromList $ catMaybe
           Just (from, to)
         reverseMapping = Map.fromList $ catMaybes $ zipWith f unsorted sorted
         g (to, _) (from, _) = if to == from then Nothing else
-          let Just movingTo = Map.lookup from reverseMapping
+          let movingTo = fromMaybe err $ Map.lookup from reverseMapping
           in Just (to, NodeInfo (From_Move from mempty) $ Just movingTo)
+        err = error "IMPOSSIBLE happens in patchThatSortsMapWith"
 
 -- | Create a 'PatchMapWithPatchingMove' that, if applied to the first 'Map' provided,
 -- will produce a 'Map' with the same values as the second 'Map' but with the