satisfy cargo clippy

[eugenebokhan]

Fixes the failing Clippy CI job (the only failing job on the scheduled master runs).

Nightly clippy's clippy::needless_late_init (part of clippy::all) now flags the late-initialized self_lt_token/self_gt_token bindings in macro/src/expand.rs, which aborts cxxbridge-macro and fails the job. Applied clippy's own suggestion: hoist the bindings into a tuple produced by the match. Behavior-preserving (the syn token fields are Copy).

Verified locally with the exact CI commands:

cargo +nightly clippy --workspace --tests --exclude demo -- -Dclippy::all -Dclippy::pedantic
cargo +nightly clippy --manifest-path demo/Cargo.toml -- -Dclippy::all

Both pass; with this lint fixed the whole workspace + tests are clippy-clean.