21.1.5.1 CreateStringIterator Abstract Operation
function CreateStringIterator(string) {
  CHECK_OBJECT_COERCIBLE(string, 'String.prototype[Symbol.iterator]');
  var s = TO_STRING(string);
  var iterator = new StringIterator;
  SET_PRIVATE(iterator, stringIteratorIteratedStringSymbol, s);
  SET_PRIVATE(iterator, stringIteratorNextIndexSymbol, 0);
  return iterator;
}