Relax some atomic barriers. Loosen up all that tension. There, doesn't that feel good?
Document task killing design and relaxed barrier rationale.
Make a forgotten assert in comm be cfg(test)-dependent
Temporary workaround to prevent taskgroup cleanup code from failing without an exception handler.
auto merge of #8195 : bblum/rust/task-cleanup, r=brson

In the first commit it is obvious why some of the barriers can be changed to ```Relaxed```, but it is not as obvious for the once I changed in ```kill.rs```. The rationale for those is documented as part of the documenting commit.

Also the last commit is a temporary hack to prevent kill signals from being received in taskgroup cleanup code, which could be fixed in a more principled way once the old runtime is gone.