Macro derive_handle_message_with_default

macro_rules! derive_handle_message_with_default {
    (
        $default_handler_method:ident,
        $(
            $message_type:ty => $handler_method:ident
        ),
        +
    ) => { ... };
}

Macro to implement a handle message trait method that delegates known types to other methods on the struct implementing ViewAssistant.