Made a small change, inspired by comments and suggestions in #35 by @astuchkin.  The change is that __iced_deferrals is now available to "custom" defer-handlers, like Rendezvous and others.  Also solves the ugly rv.__iced_deferrals.defer() hack...
made a small change for new custom defer handlers. We might want to access the `defer`
method of `Deferrals` without having to the compiler rewrite it.  So expose a _defer
method that does the same thing...
slightly better fix that the last attempt