dillon1000/react

Always reset context before beginning new work

This consolidates the reset of context to one single place right before
any new work is started.

This place is burried in a bit of an awkward place but findNextUnitOfWork
happens right before any new work starts. That way we're guaranteed to have
an empty stack before we start anything new.

The benefit of this is that we don't have to rely .return being null when
entering beginWork but we also don't have to check it every time in the
hot path.