stackitem: properly detect recursive references during serialization

If we're done with element it no longer can lead to recursion error, so fix
cases like `[arr, arr]` where we have two copies of `arr` trigger this error
for no good reason (there is no recursion there).

pkg/vm/stackitem/json.go

@@ -241,6 +241,7 @@ func toJSONWithTypes(item Item, seen map[Item]bool) (interface{}, error) {
 			arr = append(arr, s)
 		}
 		value = arr
+		delete(seen, item)
 	case *Bool:
 		value = it.value
 	case *Buffer, *ByteArray:
@@ -266,6 +267,7 @@ func toJSONWithTypes(item Item, seen map[Item]bool) (interface{}, error) {
 			})
 		}
 		value = arr
+		delete(seen, item)
 	case *Pointer:
 		value = it.pos
 	}

pkg/vm/stackitem/serialization.go

@@ -82,6 +82,7 @@ func serializeItemTo(item Item, w *io.BinWriter, allowInvalid bool, seen map[Ite
 		for i := range arr {
 			serializeItemTo(arr[i], w, allowInvalid, seen)
 		}
+		delete(seen, item)
 	case *Map:
 		seen[item] = true
 
@@ -91,6 +92,7 @@ func serializeItemTo(item Item, w *io.BinWriter, allowInvalid bool, seen map[Ite
 			serializeItemTo(t.Value().([]MapElement)[i].Key, w, allowInvalid, seen)
 			serializeItemTo(t.Value().([]MapElement)[i].Value, w, allowInvalid, seen)
 		}
+		delete(seen, item)
 	case Null:
 		w.WriteB(byte(AnyT))
 	}