Unroll :db.cardinality/many assertions

With the latest (0.3.0) DataScript, `:db.cardinality/many` attributes are returned differently than they are in Datomic.

In DataScript:

```clojure
(let [schema {:children {:db/cardinality :db.cardinality/many}}
      conn   (d/create-conn schema)]

      (d/transact! conn [{:db/id -1 :children #{"A" "B"}}])

      (->> (d/q '[:find ?e :where [?e :children _]]
                @conn)
           ffirst
           (d/entity @conn)
           :children)) ;;=> #{#{"B" "A"}}
```

whereas in Datomic the same code returns `#{"B" "A"}`.
Changing the transaction data to:

```clojure
(d/transact! conn [{:db/id -1 :children "A"}
                   {:db/id -1 :children "B"}])
```

returns `#{"A" "B"}`.

If this is the desired behavior, would you be open to a pull request that makes

```clojure
(d/transact! conn [{:db/id -1 :children #{"A" "B"}}])
```

equivalent to

```clojure
(d/transact! conn [{:db/id -1 :children "A"}
                   {:db/id -1 :children "B"}])
```