Method postorder

publication/code/chapter08/rbtree.rs:176–180

(&self)

    }

    unsafe fn postorder(&self) {
        if !self.left.is_null() { (*self.left).postorder(); }
        if !self.right.is_null() { (*self.right).postorder(); }
        println!("key: {:?}", &self.key);
    }

    unsafe fn levelorder(&self) {
        let size = self.size(0);