Update TH generation for Unit x vs (x)

See ghc/ghc#17371

patches/free-5.1.2.patch

 diff --git a/src/Control/Monad/Free/TH.hs b/src/Control/Monad/Free/TH.hs
-index 1a5ace9..856bb39 100644
+index 1a5ace9..225de20 100644
 --- a/src/Control/Monad/Free/TH.hs
 +++ b/src/Control/Monad/Free/TH.hs
-@@ -112,7 +112,13 @@ mkArg (VarT n) t
+@@ -110,9 +110,17 @@ mkArg (VarT n) t
+             [ "expected final return type `" ++ pprint n ++ "'"
+             , "but got `" ++ pprint name ++ "'"
              , "in a constructor's argument type: `" ++ pprint t ++ "'" ]
-           let tup = foldl AppT (TupleT $ length ts) ts
+-          let tup = foldl AppT (TupleT $ length ts) ts
++          let tup = case ts of
++                      [t'] -> t'
++                      _    -> foldl AppT (TupleT $ length ts) ts
            xs <- mapM (const $ newName "x") ts
 -          return $ Captured tup (LamE (map VarP xs) (TupE (map VarE xs)))
 +          return $ Captured tup (LamE (map VarP xs) (TupE (map