Panicking tasks should abort process if not handled

RFC for issue opened here: https://github.com/rust-lang/rust/issues/19610

Poisoning is not there simply to propagate panics between tasks sharing memory (it only occurs if a panic was thrown during a lock scope) and it's not there for the mutex itself. A mutex type without poisoning would be *memory unsafe* because we permit writing data structures that don't handle further use after a panic.

There would need to be many exception safety overhauls to support it and (as explained above) that has a cost. It would be difficult to find all of the code requiring changes, and this would be a *very* backwards incompatible change so it would have to be done before 1.0.