Registry flattening

This pull request flattens the registry, that is, uses dedicated public static variables instead of using string keys in `xp::$registry`:

```php
xp::$registry['errors'] => xp::$errors
xp::$registry['loader'] => xp::$loader
xp::$registry['null'] => xp::$null
xp::$registry['classpath'] => xp::$classpath
xp::$registry['cl.level'] => xp::$cll
xp::$registry['cl.inv'] => xp::$cli
xp::$registry['ext'] => xp::$ext
xp::$registry['classloader.'.$class] => xp::$cl[$class];
xp::$registry['class.'.$name] => xp::$cn[$name];
```

The function `xp::registry()` is also marked as deprecated.