Add avl tree implementation

[febo]

PR to add an AVL tree implementation using the NodeAllocator. AVL trees (theoretically) offer faster lookups as trees are more strictly balanced compared to red-black ones – although they can be slower at insert/remove operations given that they require more rotations to maintain the tree balanced.

The tests included are the same ones as the red-black tree. Adding lookup tests for both avl and red-black trees can illustrate the difference in performance:

for _ in 0..200 {
    for k in keys.iter() {
        assert!(tree.get(k) != None);
    }
}

[jarry-xiao]

This is super cool! Will take a look later today

[jarry-xiao]

Approved!