dillon1000/react

Bubble up pending work priority to the top level

This is a bit poorly structured. I'll restructure when the pieces
are better in place.

Basically we reset the priority of a node before work on the
children. The children then bump their parent if they end up
having work left.

This is the first time we're seeing deep updates happening. The
new unit test demonstrates this.

There is an interesting case that happens when we fall back out of
a deep update. We end up "completing" a node that we didn't begin.
This probably breaks in coroutines. When that completes, it'll try
to render the sibling next but that should bail out so we check
for any pending work on the sibling. That one I'm not sure about.