Reverse string slice goes into infinite loop fix

I bumped into #455 in my own code, but a little investigation showed that a small patch to transcrypt/modules/org/transcrypt/__builtin__.js (viewable as the totality of PR #702) would fix it. I don't think the patch is outsized for the increase in functionality - negative steps in slices are fairly fundamental. I also don't understand why #455 was marked 'IS:limitation'.

Thanks for the fix. I think this should be in the next major version. Still want to take a good long look at it. And indeed at least there should be a decent error report.