Add GraphTopology.incidence_cursor() for certain use-cases where the iterator is problematic

Also add raw_incidences() and step_incidence() to AppendGraph analogously to
Graph, which allows to remove RawIncidenceIterator and simplify its call sites.

src/traits.rs

@@ -5,6 +5,7 @@ use crate::maps::{EdgeMap, VertexMap};
 pub trait GraphTopology {
     type Vertex: Copy + Eq;
     type Edge: Copy + Eq;
+    type IncidenceCursor: IncidenceCursor<Self> + Copy;
 
     fn vertex_count(&self) -> usize;
     fn vertex_capacity(&self) -> usize;
@@ -20,6 +21,7 @@ pub trait GraphTopology {
     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 incidence_cursor(&self, v: Self::Vertex) -> Self::IncidenceCursor;
     fn add_vertex(&mut self) -> Self::Vertex;
     fn add_edge(&mut self, v1: Self::Vertex, v2: Self::Vertex) -> Self::Edge;
 }
@@ -28,3 +30,7 @@ pub trait GraphTopologyDeletion: GraphTopology {
     fn delete_vertex(&mut self, v: Self::Vertex);
     fn delete_edge(&mut self, e: Self::Edge);
 }
+
+pub trait IncidenceCursor<G: GraphTopology + ?Sized> {
+    fn next(&mut self, graph: &G) -> Option<(G::Vertex, G::Edge)>;
+}