simplify creating a parser from a token tree

[erickt]

Syntax extensions are pretty painful to write. One of the main reasons is that the API is pretty messy. For example, this pattern is littered across multiple syntax extensions:

    let mut p = parse::new_parser_from_tts(cx.parse_sess(),
                                           cx.cfg(),
                                           tts.iter()
                                              .map(|x| (*x).clone())
                                              .collect());

But all that line noise is pretty unnecessary. It could be easily slimmed down to:

    let mut p = cx.new_parser_from_tts(tts);

[wycats]

👍