Consider optimizations for heavily-nested scenarios

[lukewagner]

Once this proposal gets to Phase 3 and we can study the performance of realistic workloads, I think it's important for this group to measure a workload that, e.g., uses stack-switching for both generators with green-threading where there are stacks of the form:

| green-thread-handler | generator-handler 0 | ... | generator-handler N | suspend to green-thread-handler |

Because generator nesting depth is controlled by the user program, this N can be arbitrarily-large (e.g., generators can be called recursively) and a naive implementation will require an O(N) search when switching between green threads.

Assuming that the cost of this O(N) search isn't somehow amortized away, it seems like we should work out a predictable optimization that engines can perform so that, in practice, suspension to the green-thread-handler is O(1) (like a native engine would do). Although optimizing the O(N) case in general doesn't seem possible, identifying a subset of cases that cover the above scenario does seem imminently possible. However, I think it's important for us to have a proper shared discussion about what this predictable optimization is before declaring this feature finished so that (1) we can publish it in some form and continue to claim that wasm has predictable performance, (2) if there is some small tweak or addition to the proposal that would make the optimization significantly simpler or more effective, we still have the opportunity to make that tweak.