Implement String.prototype[@@iterator]

Implement `String.prototype[@@iterator]`.

http://people.mozilla.org/%7Ejorendorff/es6-draft.html#sec-string.prototype-@@iterator

Fixes issue #539
You set up the symbols before but here you didn't use them.

I'm also fine using non symbol properties as long as the property name has a trailing underscore. Using symbols is just an extra level of protection, but no real protection.