Flutterby™! : Argh Okay so after much reading it

Argh. Okay, so after much reading, it seems like the "Arena" method is the way to handle circular data structures in Rust: Keep a usize handle into a Vec<Rc<dyn ParseNode>> data structure, dereference all of those before use.

Great. Except that I add my numeric value node, get back an integer, further down dereference out of the arena to add a further integer reference, and get 'the size of `dyn ParseNode` cannot be statically determined'.

Like it's an Rc, why does it need? Sigh.