Implement Debug for the AST nodes

[angelcaru]

When I'm using a Rust library for the first time, my general learning process is usually just spraying dbg!() and todo!() everywhere to get a better feel for the datatypes I'm working with. With Syn, I can't do that because the AST nodes don't implement Debug. Is there a reason behind this? Doesn't seem hard to just add #[derive(Debug)] to every struct defintion.

[Bruflot]

This is already supported, you just need to enable the extra-traits feature: #583 (comment)