gh-110481: Implement inter-thread queue for biased reference counting (#114824)

Biased reference counting maintains two refcount fields in each object:
`ob_ref_local` and `ob_ref_shared`. The true refcount is the sum of these two
fields. In some cases, when refcounting operations are split across threads,
the ob_ref_shared field can be negative (although the total refcount must be
at least zero). In this case, the thread that decremented the refcount
requests that the owning thread give up ownership and merge the refcount
fields.

Python/object_stack.c

@@ -67,6 +67,27 @@ _PyObjectStack_Clear(_PyObjectStack *queue)
     }
 }
 
+void
+_PyObjectStack_Merge(_PyObjectStack *dst, _PyObjectStack *src)
+{
+    if (src->head == NULL) {
+        return;
+    }
+
+    if (dst->head != NULL) {
+        // First, append dst to the bottom of src
+        _PyObjectStackChunk *last = src->head;
+        while (last->prev != NULL) {
+            last = last->prev;
+        }
+        last->prev = dst->head;
+    }
+
+    // Now that src has all the chunks, set dst to src
+    dst->head = src->head;
+    src->head = NULL;
+}
+
 void
 _PyObjectStackChunk_ClearFreeList(_PyFreeListState *free_lists, int is_finalization)
 {