perf(state): const get_open

no need for additional logic, its read only anyway

The only reason for this was to keep the API with less breaking changes.
Performance is more important.

src/lib.rs

@@ -303,7 +303,7 @@ where
                 );
                 let symbol = if item.children.is_empty() {
                     self.node_no_children_symbol
-                } else if state.opened.contains(identifier) {
+                } else if state.open.contains(identifier) {
                     self.node_open_symbol
                 } else {
                     self.node_closed_symbol