dillon1000/react

Allow performWork to be called recursively

Currently we assume that performWork is never called recursively.
Ideally that is the case. We shouldn't rely on recursion anywhere
in Fiber.

However, to support the use of syncUpdates as an opt-in to forced-
synchronous updates, we need the ability to call performWork
recursively.

At the beginning of performWork, the state of the scheduler is saved;
before exiting, that state is restored, so that the scheduler can
continue where it left off. I've decided to use local variables to stash
the previous state. We could also store them in a record and push it to
a stack, but this approach has the advantage of being isolated to
performWork.