Previously it was pretty implicit where we rely on the fact that the generator
object will exist on the previous stack, and once we notice the generator
its GC handler will crawl the generator stack.

I think we also missed the generator's registers, but only the top generator
and only if it wasn't the current thread (was unable to reproduce this).

Now, keep explicit track of all the previous stacks that a thread had been on.
This cleans that up, and since things are explicit now, we can also track
where we stopped executing on each stack.  We were doing this hackily for the main
stack before, but now we can do that for generator stacks, which will let
us make those auto-growing as well.

My confidence in this change: low.