Post-order traversal over ids crashes with stack overflow on large trees

[Jörg Singer]

I fixed this issue on my fork.

Please have a look at my changes here

If you agree I would like to issue a PR on it.
On any suggestions or questions please let me know.

Br, Jörg

PS:
There are other iterators that may have the same problem. I fixed this one first because it encounters me in my project. Another similar problem seams to exist in Tree::height_of_node. This I will likely also tackle soon.