Hide __typename field if not user requested

If you request some field of interface type, we automatically add __typename to your selection, so we can use the right type. But you probably don't need that field in your structs; you can always switch on the Go type! So ideally we should omit it from the generated structs. This isn't hard in principle, it's just some awkward plumbing since the place where we need to add it and the place where we need to ignore it are not very close together.