Could add api : fork(), join() like suspend?

``` javascript
suspend(function*() {
    var fileNames = yield fs.readdir('test', suspend.resume());
    fileNames.forEach(function(fileName) {
        fs.readFile('test/' + fileName, 'utf8', suspend.fork());
    });
    var files = yield suspend.join();
})();
```

the fork() and join() are helpful.
Although the "yield resume" close but need to iterate twice
