refactor: xo-gc: + GCObjectVisitor.generation_of()

Concession to narrow MutationLogStore to only use GCObjectVisitor
instead of assuming X1Collector.

include/xo/gc/detail/IGCObjectVisitor_DX1Collector.hpp

@@ -48,6 +48,9 @@ namespace xo {
             /** allocation metadata for gc-aware data at address @p gco.
 @p gco must be the result of a call to collector's alloc() function **/
             static AllocInfo alloc_info(const DX1Collector & self, void * addr);
+            /** generation to which pointer @p addr belongs, given role @p r;
+sentinel if @p addr is not owned by collector **/
+            static Generation generation_of(const DX1Collector & self, role r, const void * addr) noexcept;
 
             // non-const methods
             /** allocate copy of source object at address @p src.