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];
xp::$registry['details.'.$name]      => xp::$meta[$name];
```

The function `xp::registry()` is also marked as deprecated.

### Performance
This shows a slight performance increase.

**Before**
```sh
OK: 5823/6403 run (580 skipped), 5823 succeeded, 0 failed
Memory used: 52530.43 kB (64897.77 kB peak)
Time taken: 19.274 seconds
```
**After**
```sh
OK: 5823/6403 run (580 skipped), 5823 succeeded, 0 failed
Memory used: 58595.12 kB (69903.66 kB peak)
Time taken: 18.550 seconds
```
*Note: The slight memory usage increase comes from TestSuite no longer releasing test classes' meta information, this may not be correct! If it is left there, the memory usage is identical.*

### BC break
This introduces a BC break, especially for the XP Compiler, which relies on `xp::$registry['details.'.$name]`.