bug 1092231 - fix traits to work with Symbols

I'm not sure there is any point in keeping this module any longer, can you just kill it and use `Symbol.iterator` [where iteratorSymbol is currently used](https://github.com/mozilla/addon-sdk/search?utf8=%E2%9C%93&q=iteratorSymbol&type=Code) ?

As of `forInIterator` it seems that it's only used in [selection](https://github.com/mozilla/addon-sdk/search?utf8=%E2%9C%93&q=forInIterator) API & I think it would make sense to just inline it there. I would also like to add a deprecation warning there suggesting to use `for of` instead so we can get rid of it entirely sometime soon.