Replace use of item maps by explicit `Index`
# Enter a commit message for this check-in. Lines beginning with # are ignored.
#
# user: fifr
# tags: graphmap
#
# EDITED     src/branching.rs
# EDITED     src/lib.rs
# EDITED     src/linkedlistgraph.rs
# EDITED     src/maxflow/dinic.rs
# EDITED     src/maxflow/edmondskarp.rs
# EDITED     src/maxflow/mod.rs
# EDITED     src/maxflow/pushrelabel.rs
# EDITED     src/mst/kruskal.rs
# EDITED     src/mst/prim.rs
# EDITED     src/mst/worstout.rs
# EDITED     src/shortestpath/bidijkstra.rs
# EDITED     src/shortestpath/dijkstra.rs
# EDITED     src/shortestpath/floydwarshall.rs
# EDITED     src/shortestpath/moorebellmanford.rs
