…ariable

_detail::ref_to_ptr<graph_type&> isn't needed because there's no potential function being stored (as with value).
Using it causes problems when passing it to CPOs because it's no longer a simple reference to the graph

Applied to incidence, neighbors, edgelist view

Use size_t for vertex_id type in examples_tests to avoid warnings

Remove Stack as template parameter because it's no longer in P1709
This allows removal of public dfs_element