binary-search-tree: Binary Search Tree Exercise

Hey I've been working on the BST exercise over at https://github.com/exercism/xcrystal/pull/72 - and while researching other implementations I noticed a lot aren't fully implementing a binary search tree.

We are only implementing and testing insertion and traversal, but not searching and deletion. A binary search tree is a great data structure to learn and implementing all the operations provide additional challenge to the programmer. Maybe we could structure the tests in approximate descending difficulty - search, traverse, insert, delete.

I also think we should separate tests which are language specific (with some sort of annotation) from the actual BST functionality (i.e. returning an enumerator).