Add EdgeMap and private generic EntityMap, update tests

src/models/graph.rs

@@ -1,6 +1,6 @@
 use typed_generational_arena::{Arena, Index};
 
-use crate::maps::VertexMap;
+use crate::maps::{EdgeMap, VertexMap};
 use crate::traits::{GraphTopology, GraphTopologyDeletion};
 
 type Vertex = Index<VertexIncidenceHeader, usize, usize>;
@@ -171,6 +171,14 @@ impl GraphTopology for Graph {
         self.incidences.len() / 2
     }
 
+    fn edge_capacity(&self) -> usize {
+        self.incidences.capacity() / 2
+    }
+
+    fn edge_map<T: Clone>(&self, default: T) -> EdgeMap<Self::Edge, T> {
+        EdgeMap::new(default, |e| e.arr_idx() / 2, self.edge_capacity())
+    }
+
     fn degree(&self, v: Self::Vertex) -> usize {
         self.vertices[v].incidence_count
     }