[slock] Slock allows sending non-Send types across thread boundaries

Slock<T> unconditionally implements Send/Sync.

Affected versions of this crate allows sending non-Send types to other threads,
which can lead to data races and memory corruption due to the data race.

References

• https://github.com/BrokenLamp/slock-rs/issues/2
• https://rustsec.org/advisories/RUSTSEC-2020-0135.html
• https://github.com/advisories/GHSA-83r8-p8v6-6gfm