implement "has_one" and "has_many" through RelatedTable and RelatedReference, anticipating unrelated references to implemented later. Can finally downcast after traversal.

[romaninsh]

let product: Table<Postgres, Product> = bakery->get_ref_as("product").unwrap();

or as an entity method:

    fn ref_products(&self) -> Table<Postgres, Product> {
        self.as_table().get_ref_as("products").unwrap()
    }

amazing!

i'm not super happy with some clone() gymnastics due to downcast_ref, but i suppose that's fine for now.