Parallel BFS order

[zommiommy]

In webgraph-algo-rs we have parallel BFS, we should update the BFS order function using the newer algorithm.

[vigna]

Not 100% sure it's a good idea. The order of a parallel BFS is unpredictable. But it might me good anyway. Certainly we must be sure to sign the number during successor enumeration. This might need using the filter function instead of the callback in some implementations.