add sim.fork_join() and fix Simulator to handle running futures with arbitrary wakers

crates/fayalite/tests/sim/expected/duplicate_names.txt

@@ -104,7 +104,6 @@ Simulation {
         did_initial_settle: true,
     },
     extern_modules: [],
-    state_ready_to_run: false,
     trace_decls: TraceModule {
         name: "duplicate_names",
         children: [
@@ -160,7 +159,12 @@ Simulation {
             },
         ),
     ],
-    instant: 1 μs,
     clocks_triggered: [],
+    event_queue: EventQueue(EventQueueData {
+        instant: 1 μs,
+        events: {},
+    }),
+    waiting_sensitivity_sets_by_address: {},
+    waiting_sensitivity_sets_by_compiled_value: {},
     ..
 }