Method inorder

publication/code/chapter08/binary_tree.rs:218–226

(&self)

    }

    fn inorder(&self) {
        if self.left.is_some() {
            self.left.as_ref().unwrap().inorder();
        }
        println!("key: {:?}", &self.key);
        if self.right.is_some() {
            self.right.as_ref().unwrap().inorder();
        }
    }

    fn postorder(&self) {
        match &self.left {