Allow recursive object types

[andreas]

This PR adds the ability to define a recursive object type: an object with a field that refers to the type itself. An example would be a tweet, which has many replies (also tweets).

To achieve this, the fields of an object is now a function that receives the type itself and must return a list of fields. The new type of obj is thus:

val obj : name:string ->
          fields:(('ctx, 'src option) typ -> ('ctx, 'src) field list) -> (* <-- new *)
          ('ctx, 'src option) typ

Example:

type tweet = {
  id : int;
  replies : tweet list;
}

let tweet = Schema.(obj
  ~name:"tweet"
  ~fields:(fun tweet -> [ (* <-- new *)
    field "id"
      ~typ:(non_null int)
      ~args:Arg.[]
      ~resolver:(fun ctx t -> t.id)
    ;
    field "replies"
      ~typ:(non_null (list tweet))
      ~args:Arg.[]
      ~resolver:(fun ctx t -> t.replies)
  ])
)

Behind the scenes, this is achieved using let rec and lazy. Thanks to @Drup for pointing me in the right direction.

closes #27

[Drup]

Nice. ;)