function copy_payload({ out, css, head, uid }) {
	const payload = new Payload();

	payload.out = out;
	payload.css = new Set(css);
	payload.uid = uid;

	payload.head = new HeadPayload();
	payload.head.out = head.out;
	payload.head.css = new Set(head.css);
	payload.head.title = head.title;
	payload.head.uid = head.uid;

	return payload;
}
Used in legacy mode to handle bindings
@param {Payload} to_copy
@returns {Payload}