[Improvement] Traversal type as option about immutable-treeutils

[lukasbuenger]

The *nodes iterator should be treated as unordered, even though that's not exactly true. It really is the only performance bottleneck in the whole library. If we gave the impression that *nodes iterates in a predictable manner, we'd lose the possibility to further optimize or completely replace the algorithm without breaking client expectations.
I did quite some benchmarking on this one and the current implementation simply represents the fastest way of iterating over all nodes (that I was able to come up with at least).

If you're simply looking for depth-first traversal, take a look at methods right and left.