Allow visits from both visitor and VisitorMut at the same time without using trait disambiguation

[docteurklein]

Closes #2067 .

Hello! I just now realize that I could have used overlapping trait Disambiguation:

let _ = Visit::visit(&statements, &mut whitelist);
let _ = VisitMut::visit(&mut statements, &mut VisitOrderBy(order));

But here I am with this proposition anyway.