Panicking tasks should abort process if not handled

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

> I promise I'm not trying to be obtuse or difficult. But AtomicPtr<T> can take arbitrary types, can it not?

It doesn't provide mutable access to arbitrary data. It allows mutation of a pointer. There is never a transaction that can be interrupted part of the way through. The mutex types are only poisoned if a panic happens *while the lock is held*.