Panicking tasks should abort process if not handled

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

> If user code is sure this is safe, why restrict them in their own usage of mutex?

It doesn't matter if the user code is safe. All that matters is unwinding goes *through* `unsafe` code *or* safe code that `unsafe` code assumes is correct. I already explained why removing the restriction isn't obvious:

> It would also force unsafe code to be less efficient, so safe code will be less efficient. There is already a high performance cost in many places due to panics. For example, PriorityQueue takes an ~30% performance hit due to exception safety. Needing to keep further method calls valid beyond just destruction will force more costs.

We're just going around in circles.