xo-gc stack: refactor + streamline.

Retiring unused Collector typealiases.
Fix #include topology.
Fix/improve write barrier setup.

include/xo/alloc2/visitor/AResourceVisitor.hpp

@@ -14,7 +14,7 @@
 #pragma once
 
 // includes (via {facet_includes})
-#include "Allocator_basic.hpp"
+#include "Allocator.hpp"
 #include <xo/facet/obj.hpp>
 #include <xo/facet/facet_implementation.hpp>
 #include <xo/facet/typeseq.hpp>

include/xo/alloc2/visitor/IResourceVisitor_Xfer.hpp

@@ -9,10 +9,18 @@
  *       [iface_facet_any.hpp.j2]
  *  3. idl for facet methods
  *       [idl/ResourceVisitor.json5]
+ *
+ *  variables:
+ *    {facet_hpp_fname} -> ResourceVisitor.hpp
+ *    {impl_hpp_subdir} -> visitor
+ *    {facet_ns1} -> xo
+ *    {facet_detail_subdir} -> visitor
+ *    {abstract_facet_fname} -> AResourceVisitor.hpp
  **/
 
 #pragma once
 
+#include "AResourceVisitor.hpp"
 #include "Allocator.hpp"
 
 namespace xo {