Previous/next card id on printings

[distributive]

It could be beneficial to have printings output the id of their neighbours. e.g.

data: {
  id: "00012"
  attributes: {
    ...
    next_printing_id: "00013",
    previous_printing_id: "00011"
  }
}

(There's probably some clearer names we could give them)

[plural]

position and position_in_set help with this somewhat.

This feels like a UI concern creeping into the API, but let's keep the issue open so we can look at it a bit down the road.