internal: add syn dependency

`syn` makes parsing Rust from proc-macros a lot simpler. `pin-init` has
not used `syn` up until now, because the Linux kernel does not support
using it. Since that changes now, we can finally utilize the added
ergonomics of parsing proc-macro input with `syn`.

Signed-off-by: Benno Lossin <lossin@kernel.org>

Author: BennoLossin

Cargo.lock

@@ -15,6 +15,7 @@ proc-macro = true
 [dependencies]
 quote = "1.0"
 proc-macro2 = "1.0"
+syn = { version = "2.0.86", features = ["full", "parsing", "visit-mut"] }
 
 [build-dependencies]
 rustc_version = "0.4"