Implement the "XML5" syntax

[kmcallister]

A parser for well-formed XML 1.0 is a very different beast from a HTML parser. But there's this XML5 spec that is quite similar to HTML parsing and could re-use a lot of the machinery, if not the actual parse rules. Servo could potentially use this for all XML parts of the Web platform. See servo/servo#3319.

[jdm]

For the record, this has been proposed as a potential project for the VALS Semester of Code, so please talk to me before starting any work on this.

[kmcallister]

Awesome! I would love to help students with html5ever projects.

[SimonSapin]

Started in https://github.com/Ygg01/xml5ever