shadow lint should use visitor or ExprUseVisitor

[Manishearth]

We're duplicating a lot of the work Visitor does.

ExprUseVisitor might work, but there's no way of handling scopes.

cc @birkenfeld thoughts?

[llogiq]

The ExprUseVisitor could at least work for the contains-check (though maybe it's even overkill for that).

[llogiq]

#449 at least uses the Visitor for the contains check.