Provide a way to compare equality of BSONValues

Even if we do extension BsonValue: Equatable {...} (which we may as well, because all of our BSON types will always be Equatable too) we can't actually make arbitrary BsonValues comparable via == because of restrictions preventing protocols from conforming to themselves, == only can only be applied to two instances of a concrete type conforming to Equatable, and not just instances of possibly different types that are both Equatable.

However, we can provide a convenience method that takes in two arbitrary {{BsonValue}}s and compares whether they are equal. To implement that we can do something like

switch left, right {

case let l as Int, let r as Int: return l == r

case let l as Double, let r as Double: return l == r

// ... all the other bson types

}

We could additionally write a custom Nimble matcher that uses this, so we could do something like expect(doc["a"]).to(bsonEqual(doc["b"])).