xo-interpreter2 stack: + reason arg to visit_gco_children()

Helps streamline DX1Collector in xo-gc/.
Want both forward and verify entry points for the same
representation.

src/object2/DInteger.cpp

@@ -34,7 +34,7 @@ namespace xo {
         }
 
         void
-        DInteger::visit_gco_children(obj<AGCObjectVisitor>) noexcept
+        DInteger::visit_gco_children(VisitReason, obj<AGCObjectVisitor>) noexcept
         {
             // no-op.  childless
         }