[NOT READY YET] [LiveHTML] domdiff changes

to @njx 

I'm going to be afk for a few hours, but I wanted to share the current state of things. The basis of the new domdiff implementation is there. Right now, there are 9 failing tests. I've been working through meaningless reorderings of edits and edge cases. After that, there's cleanup (eliminate the now-unused code, refactor redundant bits and add comments).

Feel free to comment on or hack on this until I'm back online, though I know your schedule doesn't have many openings today.