Improvement: Add a debug combinator

[cognivore]

Why?

While debugging combinatorial parsers, people often wrap a parser into debug like so:

def laLexer : Parsec Char String Unit String :=
  s.lookAhead ((s.stringP "|" <* (void c.eol <|> c.eof)) <|> s.stringP " ")

becomes

def laLexer : Parsec Char String Unit String :=
  s.lookAhead ((s.stringP "|" <* (void (c.eol >>= fun x => do
                                                     dbg_trace! x
                                                     pure x)
                                        <|> c.eof)) <|> s.stringP " ")

It's error-prone. We'd rather have this monadic bind with a trace in a separate combinator!

How?

• Write such combinator in Common.lean.

[cognivore]

Everyone who uses parsec to parse big stuff, would be very happy if this combinator was a thing.