vm: support immutable compound types

pkg/vm/stackitem/immutable.go

@@ -0,0 +1,21 @@
+package stackitem
+
+type ro struct {
+	isReadOnly bool
+}
+
+// IsReadOnly implements Immutable interface.
+func (r *ro) IsReadOnly() bool {
+	return r.isReadOnly
+}
+
+// MarkAsReadOnly implements immutable interface.
+func (r *ro) MarkAsReadOnly() {
+	r.isReadOnly = true
+}
+
+// Immutable is an interface supported by compound types (Array, Map, Struct).
+type Immutable interface {
+	IsReadOnly() bool
+	MarkAsReadOnly()
+}