Cleanup: use '_len' instead of '_size' w/ BLI API

- When returning the number of items in a collection use BLI_*_len()
- Keep _size() for size in bytes.
- Keep _count() for data structures that don't store length
  (hint this isn't a simple getter).

See P611 to apply instead of manually resolving conflicts.

source/blender/blenlib/BLI_linklist_stack.h

@@ -60,7 +60,7 @@
 } (void)0
 
 #define BLI_LINKSTACK_SIZE(var) \
-	BLI_mempool_count(var##_pool_)
+	BLI_mempool_len(var##_pool_)
 
 /* check for typeof() */
 #ifdef __GNUC__