- Refactor the stack logic into a separate data structure ChunkedStack,
  keep the push function that deals with marking as functionality specific
  to a subclass of ChunkedStack (no virtual, just reuse code).
- Add an additional (disabled) phase that maps out the reference graph
  reachable objects after the sweep phase. Prepare moving all objects in
  the small heap.

Also, rename TraceStack to TraversalWorklist because the fact
that we're using a stack is an implementation detail.