Add edge return type to GraphTopology::add_edge()

There is currently no use for the return type, but it will come.

src/traits.rs

@@ -1,5 +1,6 @@
 pub trait GraphTopology {
     type Vertex: Copy + Eq;
+    type Edge;
 
     fn vertex_count(&self) -> usize;
     fn edge_count(&self) -> usize;
@@ -8,5 +9,5 @@ pub trait GraphTopology {
     fn vertices(&self) -> impl Iterator<Item = Self::Vertex>;
     fn neighbors(&self, v: Self::Vertex) -> impl Iterator<Item = Self::Vertex>;
     fn add_vertex(&mut self) -> Self::Vertex;
-    fn add_edge(&mut self, v1: Self::Vertex, v2: Self::Vertex);
+    fn add_edge(&mut self, v1: Self::Vertex, v2: Self::Vertex) -> Self::Edge;
 }