Update to syn 2

Author: dtolnay

Cargo.toml

@@ -21,7 +21,7 @@ backtrace = { version = "0.3.51", optional = true }
 [dev-dependencies]
 futures = { version = "0.3", default-features = false }
 rustversion = "1.0.6"
-syn = { version = "1.0", features = ["full"] }
+syn = { version = "2.0", features = ["full"] }
 thiserror = "1.0"
 trybuild = { version = "1.0.66", features = ["diff"] }