Use syn and quote crates for the `match_token!` macro… … instead of the parser and quasi-quoting from Rust’s (unstable) libsyntax. This has significantly worse diagnostics when encountering unexpected syntax (e.g. no indication of which line has the offending code) but this removes all usage of unstable compiler internals that constantly need to be fixed when updating the compiler. Fixes #216. r? @nox <!-- Reviewable:start --> --- This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/html5ever/217) <!-- Reviewable:end -->