Adds .save(on:) and .delete(on:) extensions to Future

[twof]

Saving and deleting models are very common operations. This PR adds two convenience methods for doing just that.

Example usage:

return Reservation
    .query(on: databaseConnectable)
    .filter(\Reservation.user == userName)
    .first()
    .unwrap(or: Abort(.notFound, reason: "No reservations have been made"))
    .delete(on: databaseConnectable)
    .map(to: HTTPStatus.self) { _ in
        return .ok
    }

[tanner0101]

thanks :)