refactor: rename GCObject.shallow_copy -> shallow_move

resolve conflict since relying on move constructor in std_copy_for

src/object2/DList.cpp

@@ -185,7 +185,7 @@ namespace xo {
         }
 
         DList *
-        DList::shallow_copy(obj<AAllocator> mm) noexcept
+        DList::shallow_move(obj<AAllocator> mm) noexcept
         {
             return mm.std_copy_for(this);
         }