Get rid of tools

This pull request implements the part of xp-framework/rfc#287 which can be handled inside the `core` repo.
Using PHP 5.4 syntax here means we get a nasty error message when executing XP :six: with PHP 5.3:

```sh
$ XP_RT=5.3 xp -v

Parse error: syntax error, unexpected '[' in .../__xp.php on line ...
```