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.

Start by tracing how interface-type selections cause __typename to be added and where generated structs are assembled. Follow the plumbing between those stages, then verify that __typename is omitted from generated structs unless the user requested it while still supporting interface type selection.

Written by the indexing model from the issue text.