* no need to clone in hash put fn
* fmt rust code
* make code more readable
* Change vec append to extend, more friendly to rookie rust dev
* drop comment
* feat: add rust codes for avl tree
* fix a wrong usage of borrow method of RefCell
* Update avl_tree.rs
* fix comment indentation and field define of TreeNode
---------
Co-authored-by: Yudong Jin <krahets@163.com>