Investigate Overlap DescendantsWalker() and BFSWalk()

[`DescendantsWalker()`](https://github.com/heimdalr/dag/blob/1ec50306dc1a3d6440573c35729ac5fc76dff0ec/dag.go#L781) and [`BFSWalk()`](https://github.com/heimdalr/dag/blob/1ec50306dc1a3d6440573c35729ac5fc76dff0ec/visitor.go#L51) partially overlap. 

