Make the list of blocked events const

These are events that have been queued but not yet fired. There's no
reason to modify the events after creating them. Mark them as const
to ensure that doesn't happen.

src/event.cpp

@@ -291,7 +291,7 @@ void event_fire_delayed(parser_t &parser) {
     // Do not invoke new event handlers if we are unwinding (#6649).
     if (signal_check_cancel()) return;
 
-    std::vector<shared_ptr<event_t>> to_send;
+    std::vector<shared_ptr<const event_t>> to_send;
     to_send.swap(ld.blocked_events);
     assert(ld.blocked_events.empty());