RUSTSEC-2020-0059 MutexGuard::map can cause a data race in safe code

Affected versions of the crate had a Send/Sync implementation for MappedMutexGuard that only considered variance on T, while MappedMutexGuard dereferenced to U. This could of led to data races in safe Rust code when a closure used in MutexGuard::map returns U that is unrelated to T. The issue was...