Generate commitLocalPayload for elgible queries

[zth]

This generates a commitLocalPayload for any query annotated with @raw_response_type, which lets you easily commit local only data in a type safe way as if it were an actual server response. This can be used for a bunch of things, most notably local state management via Relay.

The API looks roughly like this:

module Query = [%relay.query {|
  query SomeQuery {
    loggedInUser {
      lastActiveAt
    }
  }
|}];

...

Query.commitLocalPayload(~environment, ~variables=(), ~payload={ loggedInUser: { lastActive: someTimestamp } });

Check out the added test in this diff for another example/more info.