Add GraphTopology::edges() and tests

src/models/graph.rs

@@ -181,6 +181,13 @@ impl GraphTopology for Graph {
         }
     }
 
+    fn edges(&self) -> impl Iterator<Item = Self::Edge> {
+        self.incidences
+            .iter()
+            .filter(|(i, _)| i.arr_idx() % 2 == 0)
+            .map(|(i, _)| i)
+    }
+
     fn add_vertex(&mut self) -> Self::Vertex {
         self.vertices.insert(VertexIncidenceHeader {
             incidence_count: 0,