CVE-2020-36220: Data race in va-ts

In the affected versions of this crate, Demuxer unconditionally implemented Send with no trait bounds on T. This allows sending a non-Send type T across thread boundaries, which can cause undefined behavior like unlocking a mutex from a thread that didn’t lock the mutex, or memory corruption from data race. The flaw was corrected in commit 0562cbf by adding a T: Send bound to the Send impl for Demuxer.