dillon1000/react

Rename unitOfWork -> workInProgress

These values represent fibers that are incomplete. In the current
model they're mutated in place. In a completely immutable model
they would need to be cloned for every step they make progress.
I.e. one where the child is still in this WIP state and one when
it is complete.

To clarify this I'll name them workInProgress while they're in
that state, which is also what Jordan did in his prototype.