Implicit graph helper struct

[pnevyk]

Inspired by pathfinding crate, it would be nice to have a possibility to specify a structure of an implicit graph just by providing a function to get the neighbors of a vertex. Currently, it is necessary to implement several traits (Neighbors and *Weak traits).

There could be a struct Implicit that would be a wrapper over a generic F that would represent a function implementing the Neighbors::neighbors_directed. The rest of Neighbors trait plus all *Weak traits would be implemented by the wrapper.

The implementation of Neighbors::neighbors will be probably a bit tricky, it needs to correctly handle undirected and directed graphs. And the type of Neighbors::NeighborsIter will need to be something like Box<dyn Iterator> so that the implementation of Neighbors::neighbors can chain outgoing and incoming edges.

[pnevyk]

And alternative or complement might be to implement all the mentioned traits for any appropriate F: Fn(...) -> ..., so that a function or closure could be sent directly in place of graph G.