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?

(Also, I was not suggesting that every structure need to support recovering from poisoning, just the ones that can trivially add it, such as channels and mutexes and so on. Purely as an opt-out for when a user wants tasks to fail and knows they have safely shared state. But this is just a side comment, not related to this RFC proposal.)