Finish post-order travesal with WakeupTreeIterator
The WakeupTreeIterator is now full implemented:
we now properly add nodes to the top of the
stack in reverse order, resulting in a post-order
traversal as desired.
One subtlety arose with the implementation: the
root node is not contained in the list of any
other node in the tree. To handle this issue,
a "fake" list managed by the iterator was added
into which the root is placed at construction-time.
This prevents us from needing to change any of the
logic, and we can simply treat the root as any other
node in the traversal
