Refactor ElementMap, removing its copy-on-write feature

src/lib.rs

@@ -23,14 +23,14 @@
 //! use grapherity::algorithms;
 //!
 //! let mut graph = Graph::new();
-//! // Creates a map for edge weights with a default weight.
-//! let mut weights = graph.edge_map(1);
 //! // Adds three vertices and two edges.
 //! let v1 = graph.add_vertex();
 //! let v2 = graph.add_vertex();
 //! let v3 = graph.add_vertex();
 //! graph.add_edge(v1, v2);
 //! let e = graph.add_edge(v2, v3);
+//! // Creates a map for edge weights with a default weight, after creating the edges.
+//! let mut weights = graph.edge_map(1);
 //! // Sets a non-default weight for the second edge.
 //! weights[e] = 5;
 //!
@@ -97,7 +97,8 @@
 //! graph.degree(v);
 //! ```
 //!
-//! [`ElementMap`] is not required to panic if provided with an invalid vertex or edge.
+//! [`ElementMap`] does not check validity of vertices or edges and might or might not panic in case
+//! an invalid handle is provided.
 //!
 //! Vertices and edges are provided by the graph and not intended to be generated by users.
 //!