Improve clear() method

[spinrut]

Method is currently not tail recursive; results in excessive use of stack space.

Idea for improved algorithm:

1. Perform left rotations about root until root has no left children.
2. Replace root with its right child.
3. Repeat until root is None.

[spinrut]

Clear method written in 685e753. Need to benchmark against just setting root to 'None' after implementing self-balancing.