Feat: Make HashSet usable in doc! macro

[TheDan64]

Many times I'll deduplicate i64s into a set, HashSet and I want to do:

let filter = doc! {
    "my_id": {
        "$in": id_set,
    }
};

but this doesn't work, and I have to re-collect the HashSet into a Vec. It would be nice if you could use HashSet<T> or HashSet<&T> where T is a bsonable type that normally works in Vec<T> to not have to reallocate since you're probably converting it to your own internal type anyway?

[abr-egn]

Sorry for the delay!

This does sound like a reasonable feature; we don't have much development bandwidth in the immediate future to pick this up, but we'd be happy to review a contribution if you're interested 🙂