Feature: add SSSP for types that implement `CountAllVertices` and `DijkstraUnweighted`

[bsdrks]

Single-source short-path for an unweighted graph is equivalent to DijkstraUnweighted.dijkstra called with:

• dist = vec![usize::MAX, graph.count_all_vertices()]; dist[s] = 0;
• heap = BinaryHeap::from([(Reverse(0), s)]);
• step = |acc, w| acc + w