Adaptive Radix Tree

This is an Adaptive Radix Tree implementation ported from the C implementation libart

Links:

• Adaptive Radix Tree paper: link

TODO:

• Implement a String map using the transformation described in part IV.B of the paper
• API parity with BTreeMap (or at least more of its method implemented)
• Increase test coverage -- there are many clean tests in this ART implementation
• Add benchmarks comparing against BTreeMap