Fix interface/fragment support

[mathstuf]

Currently fragment references on interfaces generates bad code. Instead of a struct with union on and the variants having fields, I get fields for each type as well as an on field. (This is when using #105.)

[tomhoule]

It should be possible to create a minimal query to reproduce the problem in the integrations test folder. I'll prioritize this above the rest when I have time to work on graphql-client (this weekend probably).

[tomhoule]

I opened a PR with a test to reproduce the problem: #153

Although apparently it's not even compiling, instead of generating bad code. Anyhow it needs to be fixed, I'll do this in priority.

[tomhoule]

#153 makes fragments work on interfaces. Support is partial because it does not work with type-refining interfaces. There is an issue (#154) for that feature.

In clear you can have a fragment that selects fields on an interface, but only the interface's own fields, not the fields of implementors. Yet.

[tomhoule]

Fixed by #153