RFC: Remove #[crate_id]

With this approach, I think it's ok to treat the `-C` flag as overriding a previous value and not worrying about needing a way to allow a list of values for a metadata attribute. I don't think we can really do that with the attribute syntax right now anyway, because a `MetaItem` can't be a bare string literal (meaning you can't say `authors("Bob", "Fred", "Joe")`). And even if we adjust the AST to allow for that, we can punt on allowing that flexibility from `-C` unless someone has a compelling reason to require it.