Panicking tasks should abort process if not handled

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

I don't think the argument for removing unwinding would be any weaker. It's very hard to see why we should pay a high compile-time cost and in many cases a significant runtime cost to support such a niche feature. These changes would make the feature more niche if anything by removing the ability to silently ignore logic errors without supervising the thread. I don't think it should be possible to do that... but it's what most people are using panics for today.