* [stdlib] Fix String.UTF16View index sharing
* [stdlib] Fix String.UnicodeScalarView index sharing
* [stdlib] Fix String.CharacterView index sharing
* [stdlib] Test advancing string indices past their ends
* [stdlib] Simplify CharacterView ranged subscript
This decreases total testing time by over a minute on my old Mac Pro.
It probably has much less effect on systems with fewer cores, but shouldn't
be any worse there.
Swift SVN r22745