Add GraphTopology::incident_edges() and tests

src/traits.rs

@@ -1,7 +1,6 @@
 use crate::maps::{EdgeMap, VertexMap};
 
 // TODO: Add functions to reserve memory for vertices and edges.
-// TODO: Add iterator of incident edges for a vertex.
 // TODO: Split out GraphTopologyAddition trait.
 pub trait GraphTopology {
     type Vertex: Copy + Eq;
@@ -19,6 +18,7 @@ pub trait GraphTopology {
     fn adjacent_vertices(&self, v: Self::Vertex) -> impl Iterator<Item = Self::Vertex>;
     fn incident_vertices(&self, e: Self::Edge) -> (Self::Vertex, Self::Vertex);
     fn edges(&self) -> impl Iterator<Item = Self::Edge>;
+    fn incident_edges(&self, v: Self::Vertex) -> impl Iterator<Item = Self::Edge>;
     fn incidences(&self, v: Self::Vertex) -> impl Iterator<Item = (Self::Vertex, Self::Edge)>;
     fn add_vertex(&mut self) -> Self::Vertex;
     fn add_edge(&mut self, v1: Self::Vertex, v2: Self::Vertex) -> Self::Edge;