Add XML fragment parsing to xml5ever

[Ygg01]

Since I deleted the xml5ever repostiory, I've decided to move this issue here. This was a feature @nox asked, and it seems that it's a pain point in servo as well see servo/servo#11995.

The algorithm for parsing xml fragment is in HTML5 spec.

Basically, what's needed is to construct a context element with given name, a NamespaceMap, and input string. It's advised to use parse_fragment from h5e as reference point.