Use daggy's StableDag

[fwcd]

Using StableDag should fix the index-shifting behavior, which previously occurred when nodes were removed from the graph.

There are a few API-breaking changes with this solution, however:

• raw_nodes is replaced by node_references
• raw_edges is replaced by edge_references
• Walker's next_node and next_edge are replaced by walk_next (as defined by the upstream API)