Update syn to v2

[Sytten]

Just opening a tracking issue since this is the way.

[tomhoule]

I don't know how much code changes that will cause, since this crate uses syn and quote way more than I'd use them if I were to write it today. But yes, that should be done and released in a new breaking version.

[Sytten]

Let me check if I can take a stab at it next week

[tomhoule]

Closed by #520