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.
Use Option .take() or .take_unwrap() instead of util::replace where possible