support for es6 Set?

I have many mongoose queries models that end up referencing an array of unique ids. An example:

``` javascript

const cursor = BidPackageModel.find({
projectId: {
$in: projectIds,
}
}).cursor()
```

where `projectids` is a set of unique ids I've built up with some little glue logic. Javascript now has a `Set` https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set

rather than having to cast a Set into a `projectids` Array , it would be awesome if mongoose was capable of supporting sets directly.

Is this something that would be realistic/feasible in terms of new features?

Start by tracing how Mongoose handles the $in value in the query shown, then identify the relevant query-casting tests or entry points. Done means a JavaScript Set can be passed directly for this use case and tests demonstrate the expected query behavior.

Written by the indexing model from the issue text.