Add FrozenGraph (without tests)

src/models.rs

@@ -1,11 +1,13 @@
 //! Concrete graph topology models.
 
 pub mod append_graph;
+pub mod frozen_graph;
 pub mod graph;
 
-// TODO: Compressed-sparse-row graph model.
-
 pub use append_graph::{
     AppendGraph, AppendGraphEdgeMap, AppendGraphIncidence, AppendGraphVertexMap,
 };
+pub use frozen_graph::{
+    FrozenGraph, FrozenGraphEdgeMap, FrozenGraphIncidence, FrozenGraphVertexMap,
+};
 pub use graph::{Graph, GraphEdgeMap, GraphIncidence, GraphVertexMap};