[event-loop] Sketch out cothreads implementation

[event-loop] Sketch out cothreads implementation

This is just an experiment.  It would let us potentially
avoid callback calling callback calling callback type code,
and instead have each handler yield when it needs to wait,
and resume where it left off when it's done waiting.

This is an initial draft.  It isn't finished being written,
has only barely been tested, etc.