xo-gc: bugfixes for GCObjectStore, unit test exapnded

In particular: drop casual assignment to DList.rest_,
will break acyclic assumption of DList.size()

xo-object2/include/xo/object2/DList.hpp

@@ -65,8 +65,8 @@ namespace xo {
 
             /** assign head **/
             void assign_head(obj<ACollector> gc, obj<AGCObject> h);
-            /** assign rest-pointer **/
-            void assign_rest(DList * r);
+            /** assign rest-pointer. Caller responsible for preserving acyclic property! **/
+            void _assign_rest(DList * r);
 
             /** pretty-printing driver; combine layout+printing **/
             bool pretty(const ppindentinfo & ppii) const;

xo-object2/src/object2/DList.cpp

@@ -128,7 +128,7 @@ namespace xo {
         }
 
         void
-        DList::assign_rest(DList * r)
+        DList::_assign_rest(DList * r)
         {
             scope log(XO_DEBUG(true), "need write barrier");