Implement String.prototype[@@iterator]

Implement `String.prototype[@@iterator]`.

http://people.mozilla.org/%7Ejorendorff/es6-draft.html#sec-string.prototype-@@iterator

Fixes issue #539
I'm not using the builtin `hasOwnProperty`, I'm using the wrapper version that calls `toProperty` before passing to the builtin version (named `$hasOwnProperty` in this file).