Initial implementation of processing

[thekid]

A processing function can handle the processing of elements passed to it in the following ways:

1. Do something with and pass it on down the chain, just like peek()
2. Do something with and return a result, just like map()
3. Decide not to pass it on, just like filter()
4. Decide it cannot be handled now, and to retry it later (no equivalent)
5. Defer its passing down the chain to a later point (no equivalent)

Example

Sequence::of($import)
  ->peek(function($record) { Console::write($record['name']); })
  ->process(function($processing, $record) {
    $existing= $this->database->lookup($record['name']);
    try {
      return $this->database->replace($existing, $record);
    } catch (InvalidRecord $e) {
      $processing->drop();
    } catch (DatabaseError $e) {
      $processing->retry($record, function() { sleep(5); });
    }
  }))
  ->peek(function($id) { Console::writeLine(' -> #', $id); )
  ->each()
;