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/depsgraph/intern/depsgraph_eval.cc

@@ -155,5 +155,5 @@ void DEG_evaluate_on_framechange(EvaluationContext *eval_ctx,
 bool DEG_needs_eval(Depsgraph *graph)
 {
 	DEG::Depsgraph *deg_graph = reinterpret_cast<DEG::Depsgraph *>(graph);
-	return BLI_gset_size(deg_graph->entry_tags) != 0;
+	return BLI_gset_len(deg_graph->entry_tags) != 0;
 }