Reimplement ARC::unwrap() and friends.
Add UnsafeAtomicRcBox::try_unwrap()
Add KillHandle and implement exit code propagation to replace join_latch
Use KillHandle in Task.
Remove join_latch
Add tests for KillHandle
Implement KillHandle::kill() and friends (unkillable, atomically). Close #6377.
Do a task-killed check at the start of task 'timeslices'.
Add BlockedTask and wake() in kill.rs.