MAINT-5351: Improve management of "current" coroutine information.

Our first cut at tracking the "current" coroutine simply reset the pointer to
NULL every time we context-switched away. But that strategy doesn't handle the
case of coroutine A launching coroutine B.
Introduce LLCoros::CoroData to track, among other things, the previous value
of the current-coroutine pointer each time we switch into a coroutine. Restore
THAT value when we switch back out.