Advisories for Cargo/Rcu_cell package

Affected versions of this crate unconditionally implement Send/Sync for RcuCell<T>. This allows users to send T: !Send to other threads (while T enclosed within RcuCell<T>), and allows users to concurrently access T: !Sync by using the APIs of RcuCell<T> that provide access to &T. This can result in memory corruption caused by data races.

Affected versions of this crate unconditionally implement Send/Sync for RcuCell<T>. This allows users to send T: !Send to other threads (while T enclosed within RcuCell<T>), and allows users to concurrently access T: !Sync by using the APIs of RcuCell<T> that provide access to &T. This can result in memory corruption caused by data races.