refactor `meta_setup`

[TimTeaFan]

The meta_setup function needs to be refactored. This should fulfill three goals:

1. The current setup uses the parent frame of the calling function. This is a dodgy implementation and should be avoided. A separate environment should be used for the setup.
2. The current implementation uses various checks even for simple setups. Refactoring meta_setup should try to speed the most common cases up. This should yield a small performance gain.
3. Refactoring meta_setup should close issue #3

At the moment I'm working on two different approaches which are implemented in the ngrp_meta_setup and the do_exit_meta_setup branches respectively.

[TimTeaFan]

Successfully implemented in the dev branch with this PR.