Support type-refining fragments

[tomhoule]

Fragments can be used to select a specific type on an interface/union.

e.g.

type Dog {
  name: String!
  hasFlappyEars: Boolean!
  hasWaggingTail: Boolean!
}

type Cat {
  meows: Boolean!
  mischievous: Boolean!
  favouriteFood: String!
}

union Pet = Dog | Cat

fragment DogAttributes on Dog {
  hasFlappyEars
  hasWaggingTail
}

# We can then query on a Pet field this way:

myPet {
  ...DogAttributes
}

# This is equivalent to:

myPet {
  ...on Dog {
    ...DogAttributes
  }
}

Relevant section of the spec

[tomhoule]

I started working on this.