Bump syn dep to the latest version (#1)

Your `Cargo.lock` is currently locking an old version of the `syn`
crate. In future versions of Rust (in particular, once
rust-lang/rust#77153) is merged), this may cause
your crate to stop compiling.

This PR bumps `syn` to the latest version in your `Cargo.lock`. This
ensures that your crate can compiler with both older and newer versions
of Rust.

Author: Aaron1011