EdenDB improvements - ref foreign child

ref_foreign_child.edl

TABLE server {
  hostname TEXT PRIMARY KEY,
}

TABLE server_volume {
  volume_name TEXT PRIMARY KEY CHILD OF server,
  mountpoint TEXT,
}

DATA server(hostname) {
  server-a WITH server_volume {
    pgtest1, '/srv/volumes/pgtest1';
  };
  server-b WITH server_volume {
    pgtest1, '/srv/volumes/pgtest1';
  };
}

TABLE db_deployment {
    deployment_name TEXT PRIMARY KEY,
}

TABLE db_deployment_instance {
    deployment_id INT PRIMARY KEY CHILD OF db_deployment,
    db_server REF FOREIGN CHILD server_volume,

    CHECK { deployment_id > 0 },
}

DATA STRUCT db_deployment [
  {
    deployment_name: foo WITH db_deployment_instance [
      {
        deployment_id: 1,
        db_server: server-a=>pgtest1,
      },
      {
        deployment_id: 2,
        db_server: server-b=>pgtest1,
      },
    ]
  }
]