Support for Traversals

@Gabriel439: Thank you [again](https://stackoverflow.com/questions/60423705/can-the-type-of-valid-graphs-be-encoded-in-dhall#comment106928373_60438316) for writing this helpful Dhall snippet. I'm bringing my further questions here to make it easier for others to find them.

Would it be possible to implement common graph traversals for this type? For instance, mapping across each vertex in an arbitrary order, or in a topological order? I'd be happy to take this on myself if you could provide pointers to literature describing how best to do so.

The issue does not name files or tests, so first locate the directed-graph type and any existing examples. Use the linked Stack Overflow discussion and traversal literature to define arbitrary-order and topological traversal; done means the project has an agreed implementation of the requested traversals.

Written by the indexing model from the issue text.