`notFollowedBy` always succeeds with parsers that don't consume input about megaparsec

[mrkkrp]

We could use solution proposed by @neongreen, but before we do, I think we should create test suite to clearly state properties of notFollowedBy in MegaParsec. Also, it's necessary to check performance of the solution, criterion can be used here to compare speed of original implementation and the proposed one.

---

Postponed until necessary testing and benchmarking are implemented.

[mrkkrp]

This is fixed now. I've decided to write the combinator on lower level, in Text.Megaparsec.Prim, this way it doesn't get hairy:

-- | @notFollowedBy p@ only succeeds when parser @p@ fails. This parser
-- does not consume any input and can be used to implement the “longest
-- match” rule.

notFollowedBy :: Stream s m t => ParsecT s u m a -> ParsecT s u m ()
notFollowedBy p = ParsecT $ \s@(State input pos _) _ _ eok eerr -> do
  l <- maybe eoi (showToken . fst) <$> uncons input
  let cok' _ _ _ = eerr $ unexpectedErr l pos
      cerr'    _ = eok () s mempty
      eok' _ _ _ = eerr $ unexpectedErr l pos
      eerr'    _ = eok () s mempty
  unParser p s cok' cerr' eok' eerr'

Since I'm doing major reorganization of the module and the solution depends on new concept of hints, the change will be committed among other changes soon (not as a separate commit).

λ> parseTest (lookAhead (string "a")) "abc"
"a"
λ> parseTest (notFollowedBy (lookAhead (string "a"))) "abc"
parse error at line 1, column 1:
unexpected 'a'
λ> parseTest (many (char 'r') *> notFollowedBy (lookAhead (string "a"))) "ra"
parse error at line 1, column 2:
unexpected 'a'
expecting 'r'
λ> parseTest (many (char 'r') <* notFollowedBy (lookAhead (string "b"))) "ra"
"r"
λ> parseTest (notFollowedBy $ notFollowedBy (string "a")) "abc"
()
λ> parseTest (many (char 'r') <* notFollowedBy eof) "ra"
"r"
λ> parseTest (many (char 'r') <* notFollowedBy eof) "r"
parse error at line 1, column 2:
unexpected end of input
expecting 'r'