DRTVWR-476: Keep coroutine-local data on toplevel()'s stack frame.

Instead of heap-allocating a CoroData instance per coroutine, storing the
pointer in a ptr_map and deleting it from the ptr_map once the
fiber_specific_ptr for that coroutine is cleaned up -- just declare a stack
instance on the top-level stack frame, the simplest C++ lifespan management.
Derive CoroData from LLInstanceTracker to detect potential name collisions and
to enumerate instances.

Continue registering each coroutine's CoroData instance in our
fiber_specific_ptr, but use a no-op deleter function.

Make ~LLCoros() directly pump the fiber scheduler a few times, instead of
having a special "LLApp" listener.