* Rename ReactFiberScheduler to ReactFiberWorkLoop
The scheduling part is mostly extracted out to the scheduler package.
What's remaining is mostly around the loop around each section of work.
I name it something with Work in it because it's very related to the
BeginWork, CompleteWork and UnwindWork sections.
* Extract throwException from UnwindWork
Our throwing works more like algebraic effects in that it's a separate
phase where we find a handler and we later unwind.
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