Implement range tree iterator

[declanvk]

Problem

• I need to iterate over subsets of a tree, instead of the entire thing.
• The solution should be more efficient than just reading all leaves and skipping those outside the range I'm interested in.

Solution Sketch

• The current tree iterator implementation uses a stack of in-progress iterators that range over the internal nodes.
  • The logic is that the tree iterator will progress the top most iterator and keep adding iterators onto the stack until it gets to a leaf node, then yield.
  • Once a node iterator is exhausted, it is popped off the stack.
• One method for implementing a ranged version of the tree iterator is to pre-processes the iterator by skipping nodes from the front and back until the keys are inside of the range requested.
  • The downside to this solution is that it could require potentially O(n) work just to create and setup the iterator.
• Another method would be to perform 2 iterative searches for the start and end of the range, and create the node iterators along the way.