vm: add InteropItem type for interop data

This is an opaque data item that is to be used by the interop functions.

pkg/vm/stack.go

@@ -92,6 +92,8 @@ func (e *Element) Bool() bool {
 			}
 		}
 		return false
+	case *InteropItem:
+		return t.value != nil
 	default:
 		panic("can't convert to bool: " + t.String())
 	}

pkg/vm/stack_item.go

@@ -248,3 +248,30 @@ func toMapKey(key StackItem) interface{} {
 		panic("wrong key type")
 	}
 }
+
+// InteropItem represents interop data on the stack.
+type InteropItem struct {
+	value interface{}
+}
+
+// NewInteropItem returns new InteropItem object.
+func NewInteropItem(value interface{}) *InteropItem {
+	return &InteropItem{
+		value: value,
+	}
+}
+
+// Value implements StackItem interface.
+func (i *InteropItem) Value() interface{} {
+	return i.value
+}
+
+// String implements stringer interface.
+func (i *InteropItem) String() string {
+	return "InteropItem"
+}
+
+// MarshalJSON implements the json.Marshaler interface.
+func (i *InteropItem) MarshalJSON() ([]byte, error) {
+	return json.Marshal(i.value)
+}