Explore possibility of generic types #415

As projects like Relay have shown, it's relatively common to repeat the same generic structures of types multiple times within a project. In the case of Relay, I'm talking about Connections.

The GraphQL definition language already has explicit support for one particular form of generic type, arrays:

type Foo {
   id: ID!
   bars: [Bar]
}

I'd like to start discussion about being able to do something similar for user-defined structures:

generic ConnectionEdge<T> {
   node: T
   cursor: String
}

generic Connection<T> {
   edges: ConnectionEdge<T>
   pageInfo: PageInfo
}

type Foo {
   id: ID!
   bars: Connection<Bar>
}

The overall goal is to reduce the amount of boilerplate in creating schemas with repetitive structures.