Add kill::Death for task death services and use it 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 (wake, try_block, etc) in kill.rs.
Change the HOF context switchers to pass a BlockedTask instead of a ~Task.
Add test::with_test_task() convenience function.
Add tests for task killing and blocking.
Stash a spare kill flag inside tasks, to save two atomic xadds in the blocking fastpath.
ignore udp tests?????