Add KillHandle and implement exit code propagation to replace join_latch
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'.
ignore udp tests?????
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.