Report panics as errors + identify panicking model

The build context is now passed as a mutable reference due to the need
to mutate data when adding a model.

Contains small unrelated cleanups and documentation improvements too.

asynchronix/tests/integration/simulation_panic.rs

@@ -0,0 +1,73 @@
+//! Model panic reporting.
+
+use asynchronix::model::Model;
+use asynchronix::ports::Output;
+use asynchronix::simulation::{ExecutionError, Mailbox, SimInit};
+use asynchronix::time::MonotonicTime;
+
+const MT_NUM_THREADS: usize = 4;
+
+#[derive(Default)]
+struct TestModel {
+    countdown_out: Output<usize>,
+}
+impl TestModel {
+    async fn countdown_in(&mut self, count: usize) {
+        if count == 0 {
+            panic!("test message");
+        }
+        self.countdown_out.send(count - 1).await;
+    }
+}
+impl Model for TestModel {}
+
+/// Pass a counter around several models and decrement it each time, panicking
+/// when it becomes zero.
+fn model_panic(num_threads: usize) {
+    const MODEL_COUNT: usize = 5;
+    const INIT_COUNTDOWN: usize = 9;
+
+    // Connect all models in a cycle graph.
+    let mut model0 = TestModel::default();
+    let mbox0 = Mailbox::new();
+    let addr0 = mbox0.address();
+
+    let mut siminit = SimInit::with_num_threads(num_threads);
+
+    let mut addr = mbox0.address();
+    for model_id in (1..MODEL_COUNT).rev() {
+        let mut model = TestModel::default();
+        let mbox = Mailbox::new();
+        model.countdown_out.connect(TestModel::countdown_in, addr);
+        addr = mbox.address();
+        siminit = siminit.add_model(model, mbox, model_id.to_string());
+    }
+
+    model0.countdown_out.connect(TestModel::countdown_in, addr);
+    siminit = siminit.add_model(model0, mbox0, 0.to_string());
+
+    // Run the simulation.
+    let t0 = MonotonicTime::EPOCH;
+    let mut simu = siminit.init(t0).unwrap();
+
+    match simu.process_event(TestModel::countdown_in, INIT_COUNTDOWN, addr0) {
+        Err(ExecutionError::Panic { model, payload }) => {
+            let msg = payload.downcast_ref::<&str>().unwrap();
+            let panicking_model_id = INIT_COUNTDOWN % MODEL_COUNT;
+
+            assert_eq!(model, panicking_model_id.to_string());
+            assert_eq!(*msg, "test message");
+        }
+        _ => panic!("panic not detected"),
+    }
+}
+
+#[test]
+fn model_panic_st() {
+    model_panic(1);
+}
+
+#[test]
+fn model_panic_mt() {
+    model_panic(MT_NUM_THREADS);
+}