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[Schema] Implementing interfaces with covariant return types.

This proposes loosening the definition of implementing an interface by allowing an implementing field to return a subtype of the interface field's return type.

This example would previously be an illegal schema, but become legal after this diff:

```graphql
interface Friendly {
  bestFriend: Friendly
}

type Person implements Friendly {
  bestFriend: Person
}
```

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