[wallnuss]

I recently spend two weeks on and off hunting down a bug on a platform that didn't support `rr`. I am fairly confident to say that if I had `rr` available it would have taken me a couple of hours at most.

Being able to run backwards from the point of failure and understand where a value is coming from is very powerful.

Having this available in Julia directly is great, and will make it much easier to get bug-reports from users.