Update to syn 2

[dtolnay]

https://github.com/dtolnay/syn/releases/tag/2.0.0

[parasyte]

@dtolnay This broke CI on one of my projects that targets Rust MSRV 1.37.0.

I have to either pin the dependency to thiserror = "=1.0.39" or hope that version 1.0.40 is yanked?

$ cargo +1.37.0 test --all
warning: unused manifest key: package.rust-version
    Updating crates.io index
  Downloaded thiserror v1.0.40
  Downloaded thiserror-impl v1.0.40
  Downloaded syn v2.0.2
error: failed to parse manifest at `C:\Users\jay\.cargo\registry\src\github.com-1ecc6299db9ec823\syn-2.0.2\Cargo.toml`

Caused by:
  failed to parse the `edition` key

Caused by:
  supported edition values are `2015` or `2018`, but `2021` is unknown