Class BinaryTree

code/chapter07/binary_tree.rs:10–14

// left 和 right 保存左右子节点链接
#[derive(Debug, Clone)]
struct BinaryTree<T> {
    key: T,
    left: Link<T>,
    right: Link<T>,
}

impl<T: Clone + Debug> BinaryTree<T> {
    fn new(key: T) -> Self {