xo-gc stack: refactor: introduce GCObjectVisitor facet

Plan using to properly level GCObjectStore and MutationLogStore
below Collector.

[WIP] not used yet

include/xo/alloc2/GCObjectVisitor.hpp

@@ -0,0 +1,22 @@
+/** @file GCObjectVisitor.hpp
+ *
+ *  Generated automagically from ingredients:
+ *  1. code generator:
+ *       [xo-facet/codegen/genfacet]
+ *     arguments:
+ *       --input [idl/GCObjectVisitor.json5]
+ *  2. jinja2 template for facet .hpp file:
+ *       [facet.hpp.j2]
+ *  3. idl for facet methods
+ *       [idl/GCObjectVisitor.json5]
+ **/
+
+#pragma once
+
+#include "gc/AGCObjectVisitor.hpp"
+#include "gc/IGCObjectVisitor_Any.hpp"
+#include "gc/IGCObjectVisitor_Xfer.hpp"
+#include "gc/RGCObjectVisitor.hpp"
+
+
+/* end GCObjectVisitor.hpp */