Make HtmlSerializer injectable.

Makes it possible to inject an HtmlSerializer via a Provider<HtmlSerializer>, so that serialization can be different on a per-request basis rather than across the board.

* Rename HtmlSerializer to HtmlSerialization, removing its abstract methods and keeping static methods handling common Html serialization mechanisms.
* Reintroduce HtmlSerializer as a pure interface.
* Move NekoSerializer to parse directory renaming it DefaultHtmlSerializer (it uses one Neko class as a helper but is not tied to Neko's parsing in any way).
* Move NekoCompactSerializer to CompactHtmlSerializer. Same argument.
* Make Provider<HtmlSerializer> injectable in GadgetHtmlParser if so chosen. If not, DefaultHtmlSerializer is always used.
* Associated changes to test code.