add debug combinator

[mattfenwick]

Purpose: add some debugging output during parsing, without changing anything else.
Something like:

def debug[E, S, T, A](parser: Parser[E, S, T, A], message: String): Parser[E, S, T, A] = {
    Parser.seq2R(Parser.bind(Parser.getState(), (s: S) => {
        println(s"state for $message: $s")
        Parser.pure(Unit)
      }), parser)
  }

Questions:

• how to test whether output has been written to console?
• should the token state and state both be dumped to console? Should there be extra, optional arguments to control that?
• what's the best argument order?

First pass:

// b -> Parser e s (m t) a -> Parser e s (m t) a
function debug(message, parser) {
    return seq2R(bind(getState, (s) => console.log("state for " + message + ": " + s)),
                      parser);
}