Don't use hints with custom error messages

Close #75.

Now accumulated hints are not used with ‘ParseError’ records that have
only custom messages in them (created with ‘Message’ constructor, as
opposed to ‘Unexpected’ or ‘Expected’). This strips “expected” line from
custom error messages where it's unlikely to be relevant anyway.

Text/Megaparsec/Prim.hs

@@ -136,9 +136,18 @@ toHints err = Hints hints
         msgs  = filter ((== 1) . fromEnum) $ errorMessages err
 
 -- | @withHints hs c@ makes “error” continuation @c@ use given hints @hs@.
+--
+-- Note that if resulting continuation gets 'ParseError' where all messages
+-- are created with 'Message' constructor, hints are ignored.
 
 withHints :: Hints -> (ParseError -> m b) -> ParseError -> m b
-withHints (Hints xs) c = c . addErrorMessages (Expected <$> concat xs)
+withHints (Hints xs) c e =
+  let isMessage (Message _) = True
+      isMessage _           = False
+  in (if all isMessage (errorMessages e)
+      then c
+      else c . addErrorMessages (Expected <$> concat xs))
+     e
 
 -- | @accHints hs c@ results in “OK” continuation that will add given hints
 -- @hs@ to third argument of original continuation @c@.