Outdated comment incorrectly describes debug state slots as not compatible with moving GC

[alexcrichton]

Right now debug state slot management is documented as not supporting moving GCs, and at the time of creation we indeed didn't have a moving GC. Nowadays though with copying as the default collector we do indeed have a moving GC, so either this comment needs updating or the implementation needs updating as well.

[cfallin]

(hi from PTO -- just dropping in briefly)

That's an outdated comment fortunately; the values in debug state slots are fully rooted and will be updated by a moving GC. I'll put this on my todo list to update that comment when I'm back.