New expressions for getting top/bottom part of an array

Description of Linked Ticket

It is rather awkward that to get topN array elements, you would do $sortArray+$slice while there's topN accumulator available.

Proposing: 

$topN: {input: <expression resolving to array>, n: <n expression>, sortBy: <spec>}

e.g.:

[
  {
    $set: {
      topThreeOptions: {
        $topN: {
          input: "$options",
          n: 3,
          sortBy: { votes: -1 },
        },
      },
      fiveWatchersAZ: {
        $topN: {
          input: "$watchers",
          n: 5,
          sortBy: 1
        },
      },
    },
  },
]