Migrate to cargo_embargo. am: 5af18216f9 am: 084f59051d am: d7c9ce1d44

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/syn/+/2854959

Change-Id: Ibb8e5ba6b3e1de55e604faf055928477dd970e1f
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>