AVL tree implementation

[saltyJeff]

I've implemented a version of this library that replaces the linked list with an AVL tree for managing registers. This changes reads / updates from O(n) to O(log N) which has helped my code a lot.

https://github.com/UCLA-Rocket-Project/daq-slave/tree/master/lib/Modbus

Should I submit a PR? This does have the side effect of increasing code size / bumping insertions to O(log N).