Experiment with a more consistent (T)ParentNode only interface.

[ianjosephwilson]

I'm not sure this is what we want yet. I put it together for #94 but maybe other things need to line up before we're ready to act on this. Specifically the instance check in def html() seems not ideal if we are calling html() all over the place but if we're only calling it a few times at the top-level then maybe its fine.

[davepeck]

Yeah the main thing that keeps holding me back here is the need to wrap so much more in Fragment; I can't decide if it defies expectations, or whether those expectations even matter outside of a handful of unit tests. And I'm not sure about that instance check; perhaps we can avoid it in more cases.