Changes since 0.2.0:
- Fix a rare case where `SharedThread::is_finished` could block: if the
  thread has a thread-local destructor that runs for a long time, and
  another thread is already trying to join it.
- Document that the join functions might block longer than expected in
  that case.
- Document the pre-existing panic in `join_timeout` if `Instant`
  addition overflows.