Allow __all to request all fields

A fun thing we can do, because we generate code, is to allow you to put in a pseudo-field like `__all`, which we expand to all the fields. That's probably a terrible idea, both in that it diverges from the spec, and you shouldn't want to do that, but we could do it anyway! And it would be useful especially for tests.

The issue names no implementation files or tests. Start by tracing how GraphQL field selections are expanded during code generation, then determine how a __all selection should identify every field without violating existing behavior. Done means __all expands to all fields and the behavior is covered by tests, especially the stated test use case.

Written by the indexing model from the issue text.