Migrate to cargo_embargo. am: 5af18216f9 am: 10d9340ef9 am: b16bbb64a8

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

Change-Id: I92de0ebb6d95334b65f8e311fd2dd0971b5b33be
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>