Traversing example in Scala, Groovy, Kotlin

Add example of using this library API in Scala, Groovy, Kotlin as test or sample code or code snippet.

There is no required minimum, even one example from list is good to have:
- iterators using DFS/BFS pre/post order
- DFS pre-order and custom visitor
- DFS post-order and custom visitor
- BFS pre-oreder and custom visitor
- BFS post-order and custom visitor

The description is intentionally vague to test existing documentation.

Feel free to open issues against documentation and samples.