dillon1000/react

Reorganize the top level around a FiberRoot

We need a canonical stateful root for each. I don't really want to
overload the HostContainer for this purpose since it makes the
fiber code more specialized.

Instead I create a root which represents an actual stateful root.

When these get scheduled they get chained together in a linked
list. However, we don't hold onto anything that doesn't have
scheduled work. This will help us release everything automatically
in the GC, as long as there are no subscriptions nor scheduled
work.