Add EdgeMap and private generic EntityMap, update tests

src/traits.rs

@@ -1,4 +1,4 @@
-use crate::maps::VertexMap;
+use crate::maps::{EdgeMap, VertexMap};
 
 // TODO: Add functions to reserve memory for vertices and edges.
 // TODO: Add iterator of incident edges for a vertex.
@@ -12,6 +12,8 @@ pub trait GraphTopology {
     fn vertex_capacity(&self) -> usize;
     fn vertex_map<T: Clone>(&self, default: T) -> VertexMap<Self::Vertex, T>;
     fn edge_count(&self) -> usize;
+    fn edge_capacity(&self) -> usize;
+    fn edge_map<T: Clone>(&self, default: T) -> EdgeMap<Self::Edge, T>;
     fn degree(&self, v: Self::Vertex) -> usize;
     fn are_adjacent(&self, v1: Self::Vertex, v2: Self::Vertex) -> bool;
     fn vertices(&self) -> impl Iterator<Item = Self::Vertex>;