xo-reader2: + SyntaxStateMachine.forward_children method

Support gc traversal with goal of making ParserStateMachine a gc root

src/reader2/DQuoteSsm.cpp

@@ -211,6 +211,12 @@ namespace xo {
                                              refrtag("expect", this->get_expect_str()));
         }
 
+        void
+        DQuoteSsm::forward_children(obj<ACollector> gc) noexcept
+        {
+            gc.forward_pivot_inplace(&expr_);
+        }
+
     } /*namespace scm*/
 } /*namespace xo*/