Support relative URLs for unknown schemes. Please review! Fixes https…

…://www.w3.org/Bugs/Public/show_bug.cgi?id=27233.

This makes a great many changes:
* scheme data -> path[0] + non-relative flag
* Introduce “is special” check for URLs
* Rewrote “URL writing” to account for wider set of conforming URLs
* API no longer allows setting scheme across special URL boundary
* Remove null from host serializer
* Remove empty string handling from host parser (only needed for
special URLs, done inline in the parser)
* Restricted special backslash handling to special URLs
* Added new states to the parser and renamed some existing states.
* Rewrote URL’s serializer algorithm.