xo-gc stack: refactor + streamline.

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

include/xo/numeric/detail/INumeric_Xfer.hpp

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