This adds an edge_entry() method to GraphMap's public interface along with
a thin wrapper over an IndexMap entry.

Differences with the wrapped entry:

• source(): returns the source edge, replacing key()
• target(): returns the target edge, replacing key()

Note that this enables merging parallel edges, which currently requires quite a
ceremony.

For example, when counting occurrences:

// before
if let Some(old) = gr.add_edge("x", "y", 1) {
    gr.add_edge("x", "y", old + 1);
}

// after
*gr.edge_entry("x", "y").or_insert(0) += 1;