Extend `Neighbors` trait with common aliases

[pnevyk]

Add new methods listed below to Neighbors trait. All of them are aliases to an already-provided functionality so they should have a default implementation. These methods should also be added to graph encapsulations in graph module.

• successors(&self, src: &Self::VertexIndex) -> Self::NeighborsIter<'_> aliased to self.neighbors_directed(src, Outgoing)
• predecessors(&self, src: &Self::VertexIndex) -> Self::NeighborsIter<'_> aliased to self.neighbors_directed(src, Incoming)
• out_degree(&self, src: &Self::VertexIndex) -> usize aliased to self.degree_directed(src, Outgoing)
• in_degree(&self, src: &Self::VertexIndex) -> usize aliased to self.degree_directed(src, Incoming)