dillon1000/react

Move wip fibers to childInProgress

This took a while to figure out, but we need to be able to store
children that are currently being worked on separately from the
current children. We always need a canonical "current" children
so that we can update them. However, we also need a different set
that we're currently working on so that we have a way to get to
already progressed work.

This solve the starvation problem in the first render because now
we can reach children that were never rendered and have a place
to store their progressed work on. The unit test changes tests
this.

This lets us get rid of the hasWorkInProgress flag.

When we reconcile new children we need to reconcile them against
progressed work so that we can reuse it. The progressed work is
"work in progress" nodes. So in that case we need to mutate
instead of clone, to preserve the invariant that only two versions
exist at any given point. This effectively forks the
ReactChildFiber implementation.