Add GraphTopology::edges() and tests

src/models/append_graph.rs

@@ -102,6 +102,10 @@ impl GraphTopology for AppendGraph {
         }
     }
 
+    fn edges(&self) -> impl Iterator<Item = Self::Edge> {
+        (0..self.incidences.len()).step_by(2).map(Incidence)
+    }
+
     fn add_vertex(&mut self) -> Self::Vertex {
         self.vertices.push(VertexIncidenceHeader {
             incidence_count: 0,