upgrade syn from v1 to v2

[jan-ferdinand]

About two months after work on tasm-lang has started, its dependency syn released a new major version, v2.0.¹ It “contains a batch of syntax tree improvements to incorporate ongoing Rust language development from the past 3.5 years since syn 1” [0], i.e., reflects current-day Rust more accurately.

While the list of breaking changes [0] is long, postponing the upgrade of a crate tasm-lang depends upon so heavily might cause even bigger headaches in the future.

[0] https://github.com/dtolnay/syn/releases/tag/2.0.0

Footnotes

1. latest: v2.0.48 ↩