AVL Trees

These are better versions of Binary Search Trees. The code only impliments AVL Trees for in-memory usage. It doesn't do disk storage (yet). the functions you can use are:

1. Insert
2. Search
3. Update
4. Delete

Refer to main.go for how to use them. And refer to avl.go for their code.

Time Complexities:

1. Insert: O(log₂ N)
2. Delete: O(log₂ N)
3. Update: O(log₂ N)
4. Search: O(log₂ N)