xo-gc stack: refactor: introduce GCObjectVisitor facet

Plan using to properly level GCObjectStore and MutationLogStore
below Collector.

[WIP] not used yet

src/alloc2/CMakeLists.txt

@@ -9,7 +9,9 @@ set(SELF_SRCS
     CollectorTypeRegistry.cpp
 
     facet/ICollector_Any.cpp
+
     IGCObject_Any.cpp
+    facet/IGCObjectVisitor_Any.cpp
 
     AAllocator.cpp
     IAllocator_Any.cpp

src/alloc2/facet/IGCObjectVisitor_Any.cpp

@@ -0,0 +1,48 @@
+/** @file IGCObjectVisitor_Any.cpp
+ *
+ **/
+
+#include "gc/IGCObjectVisitor_Any.hpp"
+#include <iostream>
+#include <exception>
+
+namespace xo {
+namespace mm {
+
+using xo::facet::DVariantPlaceholder;
+using xo::facet::typeseq;
+using xo::facet::valid_facet_implementation;
+
+void
+IGCObjectVisitor_Any::_fatal()
+{
+    /* control here on uninitialized IAllocator_Any.
+     * Initialized instance will have specific implementation type
+     */
+    std::cerr << "fatal"
+              << ": attempt to call uninitialized"
+              << " IGCObjectVisitor_Any method"
+              << std::endl;
+    std::terminate();
+}
+
+typeseq
+IGCObjectVisitor_Any::s_typeseq = typeseq::id<DVariantPlaceholder>();
+
+bool
+IGCObjectVisitor_Any::_valid
+  = valid_facet_implementation<AGCObjectVisitor, IGCObjectVisitor_Any>();
+
+// nonconst methods
+
+auto
+IGCObjectVisitor_Any::visit_child(Opaque, AGCObject *, void **)  const  noexcept -> void
+{
+    _fatal();
+}
+
+
+} /*namespace mm*/
+} /*namespace xo*/
+
+/* end IGCObjectVisitor_Any.cpp */