[RFC-2] Implement a model generator

[maticzav]

Currently, SwiftGraphQL only generates the layer that lets you express GraphQL queries using Swift language. It is widespread for developers to reuse some part of their schema in their application code.

We want to keep the model and the schema separated in the codebase. To get the best of both worlds, however, we could create a generator that would read the schema and generate boilerplate code that you may copy-paste into your Swift project.

It would contain the structure definition as well as the basic one-to-one selection set of all the fields in the model. To portray the idea, consider the following model:

type Human {
  id: ID!
  name: String!
}

This would generate

struct Human {
   let id: String
   let name: String

   static let selection = Selection<Human, Objects.Human> {
      Human(
         id: try $0.id(),
         name: try $0.name()
      )
   }
}