Feature: Distribute work

This PR allows distributing elements from a sequence on workers:

```php
$workers= [
  new LocalWorkerProcess('com.example.parallel.ImageResizer', [800, 600]),
  new LocalWorkerProcess('com.example.parallel.ImageResizer', [800, 600])
];

$results= Sequence::of($files)
  ->filter(new ExtensionEqualsFilter('.jpg'))
  ->map(function($e) { return $e->getUri(); })
  ->distribute(new WorkerProcesses($workers))
  ->each('util.cmd.Console::writeLine')
;

foreach ($workers as $worker) {
  $worker->shutdown();
}
```