Generic type variables

This pull request implements xp-framework/rfc#214 and 282 generic type variables

## Example

```php
#[@generic(self= 'T')]
class Unserializer extends \lang\Object {

  /**
   * Returns a list of all elements
   *
   * @param   var $arg
   * @return  T element
   */
  #[@generic(return= 'T')]
  public function newInstance($arg= null) {
    return null === $arg ? $T->default : $T->newInstance($arg);
  }
}
```