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/examples/assembly.rs

@@ -85,7 +85,7 @@ impl Model for MotorAssembly {}
 impl ProtoModel for ProtoMotorAssembly {
     type Model = MotorAssembly;
 
-    fn build(self, ctx: &BuildContext<Self>) -> MotorAssembly {
+    fn build(self, ctx: &mut BuildContext<Self>) -> MotorAssembly {
         let mut assembly = MotorAssembly::new();
         let mut motor = Motor::new(self.init_pos);
         let mut driver = Driver::new(1.0);

asynchronix/examples/external_input.rs

@@ -58,7 +58,7 @@ impl ProtoModel for ProtoListener {
     type Model = Listener;
 
     /// Start the UDP Server immediately upon model construction.
-    fn build(self, _: &BuildContext<Self>) -> Listener {
+    fn build(self, _: &mut BuildContext<Self>) -> Listener {
         let (tx, rx) = channel();
 
         let external_handle = thread::spawn(move || {