code: function groupByPlacement(bidsByPlacement, bid) {
    if (!bidsByPlacement[bid.adUnitCode]) { bidsByPlacement[bid.adUnitCode] = { bids: [] }; }
    bidsByPlacement[bid.adUnitCode].bids.push(bid);
    return bidsByPlacement;
  }
docs: groupByPlacement is a reduce function that converts an array of Bid objects
  to an object with placement codes as keys, with each key representing an object
  with an array of `Bid` objects for that placement
  @returns {*} as { [adUnitCode]: { bids: [Bid, Bid, Bid] } }