Fix string of Range, SizedRange .iterator(), .by()

The previous implementation would for example return

    1..10.iterator()

which looks like it should be valid Ceylon code to reproduce it, but
isn't because you need to wrap the range in parentheses:

    (1..10).iterator()
Add string to iterables and iterators
Merge pull request #489 from lucaswerkmeister/iterableStrings

Better iterable strings