Method contains

code/chapter08/graph_adjlist.rs:81–87

(&self, key: &T)

    }

    fn contains(&self, key: &T) -> bool {
        for (nbr, _vertex) in self.vertices.iter() {
            if nbr == key { return true; }
        }

        false
    }

    fn add_vertex(&mut self, key: &T) -> Option<Vertex<T>> {
        let vertex = Vertex::new(key.clone());