Handle interfaces correctly

[numso]

Looks like we don't parse and validate interfaces correctly. The following query should work on the Github API:

  query "GetRepo" do
    repository(owner: ^owner, name: ^name) do
      owner do # this line fails
        id
      end
    end
  end

Instead, we get Did not expect to find a subquery on Repository.owner.

Owner has the type:

{
  "kind": "NON_NULL",
  "name": null,
  "ofType": {
    "kind": "INTERFACE",
    "name": "RepositoryOwner",
    "ofType": null
  }
}