Unsound sending of non-Send types across threads in threadalone
Affected versions can run the `Drop` impl of a non-Send type on a different
thread than it was created on.The flaw occurs when a stderr write performed by the `threadalone` crate fails,
for example because stderr is redirected to a location on a filesystem that is
full, or because stderr is a pipe that has been closed by the reader.Dropping a non-Send type on the wrong thread is unsound. If used with a type
such as a pthread-based `MutexGuard`, [the consequence is undefined
behavior][mutexguard]. If used with `Rc`, there would be a data race on the
reference count, which is likewise undefined behavior.