Method add

code/chapter09/conshash.rs:59–64

(&mut self, node: &T)

    }

    fn add(&mut self, node: &T) {
        for i in 0..self.replicas {
            let key = hash(&format!("{}{}", node.to_string(), i.to_string()));
            self.ring.insert(key, node.clone());
        }
    }

    // 批量删除结点
    fn remove_multi(&mut self, nodes: &[T]) {