GraphIterator trait

[pnevyk]

GraphIterator trait would be a variation on Iterator from the standard library, requiring to implement just "next" functionality (probably next_vertex and next_edge, but I haven't thought about the details at all yet) and providing iterator-like functionality that makes sense to do for graphs. A list of such functionality (not exhaustive):

• map(v, e), map_vertices(v), map_edges(e)
• filter(v, e), filter_vertices(v), filter_edges(e)
• filter_map
• count -> (v, e), vertex_count -> v, edge_count -> e
• collect -> G with G generic type implementing a trait

When consuming, vertices are always processed first and edges second (existence of an edge is dependent on its endpoint(s) not being removed), which may impose some implementation challenges.