rr 5.6.0 released

[Robert O'Callahan]

Keno's preparing to land a major change to how syscallbuf handles signal interruptions, so I thought we should do a (long overdue) release now before that happens.

https://github.com/rr-debugger/rr/releases/tag/5.6.0

I think we've had pretty good testing lately, partly thanks to the new CI (thanks for that, Julia team!). I built release packages on Ubuntu 18 LTS and tested them there and they worked first time so I'm confident it's solid.

Thanks!!!

Rob

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