Provide a method to get each fragment

If you have a query like
```graphql
query Q {
f { # type: I (interface)
...Fragment # on I (or J which implements I)
}
}
```
Now, `resp.F` is a `QFI` which implements `Fragment`. But the concrete type is, say, `*QFImpl` which embeds a `*FragmentImpl`. Say you want to call some function that wants a `Fragment`; you pass it `resp.F` and that works. But now say it wants to type-switch; it might think it can switch on e.g. `*FragmentImpl`, but actually you passed it a `*QFImpl`, so it can't! (It may not even know about `QFI`!)

Right now what you probably have to do is switch on all the implementations, and extract their embed `.FragmentImpl`, and pass those, what a mess. Instead we could put on `QFI` a method `GetFragment() Fragment` just like we do for other shared fields. In practice this will mostly be obviated by #30, after which you can dispense with `QFI`/`*QFImpl` entirely, but it may be worth adding anyway.

Start by tracing how QFI and other shared-field methods are generated, then compare the Fragment and FragmentImpl handling described in the issue. Check issue #30 before proceeding; done means the generated interface provides access to the underlying Fragment without requiring callers to switch over every concrete implementation.

Written by the indexing model from the issue text.