Rename `VertexAroundVertexIterator` to `Neighbors`

[cactusdualcore]

Given a vertex, all vertices sharing a common edge with it are its neighbors. Additionally, this name is more concise and harder to confuse with, e.g. HalfedgeAroundVertexIter or VertexAroundFaceIter.

[cactusdualcore]

Something similar could be done with HalfedgeAroundFaceIter as well, because the half edges around any face constitute the face's perimeter. So it could be renamed to PerimeterHalfedges, again making confusion more unlikely.