This PR contains #345 and #333.

The differences with #345:

• optimizations: from my profilings we went from a 25% slowdown on factories to 0%, so 5.2 should not be slower than 5.1 (that was the main blocker for me)
• factories can inject a new object (DI\Factory\RequestedEntry) to know the requested definition name: that replaces the injection of Definition instances. The reason for that is to avoid coupling user factories with definitions which are an implementation detail of PHP-DI. If definitions ever change, it will not break factories.

Example to get the requested entry name:

factory(function(ContainerInterface $c, RequestedEntry $e) {
    return new Thing($c->get($e->getName()));
});

Example to inject other services:

factory(function(Logger $logger, Mailer $mailer) {
    return new Thing($logger, $mailer);
});

TODO:

• update changelog
• documentation

This is the last feature planned for 5.2, after that it's release time!