Panicking tasks should abort process if not handled

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

Not poisoning mutable shared memory implies either removing unwinding support or forcing all code to be exception safe to the point where it isn't memory unsafe. This would cause further performance regressions and complexity in low-level code beyond the high costs that are already paid to support unwinding.

At that point, forcing exceptions to be caught only at thread boundaries would no longer be justifiable. It would be entirely correct to make a safe API for catching exceptions *anywhere* and it would then be used as another runtime error handling mechanism. I don't think that's what most people want but it's the inevitable result of removing poisoning.