SPEC: Async code execution

# Some readings before :

* https://davidwalsh.name/async-generators
* https://www.promisejs.org/generators/
* https://github.com/tj/co
* https://github.com/laverdet/node-fibers

# Purpose :

- Be able to natively support asynchronous calls in a synchronous syntax
- Be able to handle promises natively in a synchronous manneer

Should be able to write :

```php
then(function($result) {
echo "$result\n";
echo "**done**\n";
});
```