Resolve conflicts of MapNode with duplicate key caused by concurrent update to the same key with different type values other than primitive

[TanUkkii007]

Document state to JSON conversion introduced from #12 does not handle duplicate key as reported at this comment.

Duplicate key can be made when MapNode is updated with different type values other than primitive.

For example, concurrently updating to the same key "key" with object and string

val p1 = p0
    .applyCmd(doc.downField("key") := `{}`)
    .applyCmd(doc.downField("key").downField("key2") := "D")
val q1 = q0.applyCmd(doc.downField("key") := "C")
merge(p1, q1)

results in duplicate keys, RegT(StrK(key)) and MapT(StrK(key)).

MapNode(
  Map(
    MapT(DocK) -> MapNode(
      Map(
        RegT(StrK(key)) -> RegNode(Map(Id(2,q) -> Str(C))),
        MapT(StrK(key)) -> MapNode(Map(RegT(StrK(key2)) -> RegNode(Map(Id(3,p) -> Str(D)))),Map(StrK(key2) -> Set(Id(3,p))))
      ),
      Map(StrK(key) -> Set(Id(2,p), Id(3,p), Id(2,q)))
    )
  ),
  Map(DocK -> Set(Id(1,p), Id(2,p), Id(3,p), Id(2,q)))
)

[TanUkkii007]

I suspect ListNode might have duplicate index as well as MapNode but have not reproduced it yet. Lemma 4 says that concurrent insertion to the same cursor will result in deterministic order so ListNode will not have duplicate indexes by INSERT. Lemma 7 says that concurrent ASSIGN operation with non-primitive values to the same position will have different keys. Checking ListNode state under concurrent assignment to the same position would help our understandings.