Fix Permutation's next function to work with Filter

[astrieanna]

See #4381

Filter does not work properly with Permutation because the next function modifies its arguments.
Adding a call to copy fixes this by not modifying the second argument.

[StefanKarpinski]

It would be really nice to not have to copy this every iteration – that's a total performance killer.

[astrieanna]

I don't know how else to maintain the assumption that next(itr,s) will not modify s.
Or is that not valid assumption, and I should instead be changing filter?

I guess it also makes no sense to have modifying (next!) and non-modifying (next) versions, since the second argument in many cases is an Int, which can't really have a next! function anyway. (Unless it's ok for the ! to mean "I might or might not modify the arguments, since I might not be able to, but I will return the right answer")

[StefanKarpinski]

No, it's a problem – I want the intuitive behavior, I just don't want to give up the performance. See the previous discussion here, and related pull request discussions: #4055 and #4136. I guess it was a bit optimistic to think that mutating this thing wouldn't come back to bite us. cc: @kmsquire