Method add_neighbor

code/chapter08/graph_adjlist.rs:28–30

(&mut self, nbr: T, wt: i32)

    }

    fn add_neighbor(&mut self, nbr: T, wt: i32) {
        self.connects.push((nbr, wt));
    }

    // 获取相邻的点集合
    fn get_connects(&self) -> Vec<&T> {