[internment] Use after free in internment

An issue was discovered in the internment crate through 2020-05-28 for Rust. ArcIntern::drop has a race condition and resultant use-after-free.

References

• https://nvd.nist.gov/vuln/detail/CVE-2020-35874
• https://github.com/droundy/internment/issues/11
• https://rustsec.org/advisories/RUSTSEC-2020-0017.html
• https://github.com/advisories/GHSA-96w3-p368-4h8c