Memory corruption via unchecked ARefss mapping

Memory-safety failure in the reffers crate before 0.6.1 allows local users to corrupt memory through ARefss::map(). ARefss::map() accepts a target type without Send and Sync bounds even though ARefss implements both traits, permitting a non-thread-safe object to cross threads. Exploitation requires a consuming Rust program to invoke the unsound mapping and access the resulting object concurrently; the demonstrated consequence is a process crash.