Don't build a new Set when parsing an extension tag

Sets are not terribly cheap yet, and this set is really small. Just test the
three cases explicitly.

Change-Id: I08d2acafe29153012be7b81b1d7ec4b3c2fcf38a
More performance tweaks / tokenizer cleanup

* Remove old and unused 'new String' logic
* simplify, optimize and partially inline inline_breaks
* Remove some dead code from the tokenizer

Change-Id: Idccadc5ef891d3b622b316060ed537a18fcac772