Support interfaces

We have some support for interfaces, but it's not at all clear how you'd implement a handler for something like:

```graphql
type Query {
  dog: Dog
} 

type Dog {
  name: Text!
  owner: Sentient
}

interface Sentient {
  name: Text!
}

type Human implements Sentient {
  name: Text!
}
```

What would the Haskell implementation for `dog.owner` look like?