Provide more ways to customize CPO `edges(g, uid)`

[akrzemi1]

Currently, CPO edges(g, uid) has only two ways of customizing it:

1. Either by providing a free function edges(g, uid)
2. Or when vertex_t<G> is a forward_range and find_vertex(g, uid) is valid.

This is the nly CPO that does not allow a customization via a member function. Consider allowing one or both of the following customizations:

3. Return g.edges(uid) if valid.
4. Return edges(g, g.vertices()[uid]) if valid and decltype(g.vertices()) is random_access_range.