explicit is better than implicit

dropped dead code

prefer sorted Map/Set for canonical order of results -- pass ordering via fresh copy of empty; discontinued map_nodes, del_nodes conveniences -- avoid inefficient mapValues wrapper (this is not Table.map from ML); tuned signature;

tuned imports;

tuned signature;

discontinued obsolete Graph.all_paths (last seen in 1524d69783d3 and AFP/80bbbdbfec62);

merged