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.
Add option::take(), the building block of the option::take_* family.
Replace *rust_task ptrs in taskgroup code with TaskHandle, for transitioning to newsched killing.
(cleanup) Modernize taskgroup code for the new borrow-checker.
(cleanup) Don't check taskgroup generation monotonicity unless cfg(test).
(cleanup) impl TaskSet
Fix linked failure tests to block forever instead of looping around yield.
Enable taskgroup code for newsched spawns.
Rename TCB to Taskgroup
Add watched and indestructible spawn modes.
Fix warnings in src/test/bench tests. Nobody will ever care.
Fix warnings in stdtest and extratest. Maybe somebody will care.