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/src/ports.rs

@@ -69,7 +69,7 @@
 //! impl ProtoModel for ProtoParentModel {
 //!     type Model = ParentModel;
 //!
-//!     fn build(self, ctx: &BuildContext<Self>) -> ParentModel {
+//!     fn build(self, ctx: &mut BuildContext<Self>) -> ParentModel {
 //!         let mut child = ChildModel::new(self.output.clone());
 //!
 //!         ctx.add_submodel(child, Mailbox::new(), "child");