convert explicit And/Or/Not into unified Operator type

Created by: camdencheek

This is a first step towards resolving the @rvantonder's feedback here.

Essentially, it unifies the And, Or, and Not types into a single operator type. This makes it easier to do things like query optimization, modification, etc. at the cost of being very slightly more verbose.

I'm honestly not sure what the next step will be, since I don't think we can unify the Atom variants under a single type like we do with our query tree since each variant has different required parameters. @rvantonder I'd be interested in pairing on this if you see an opportunity to go further with this.