Checker: Redesign checker runner to library

[F30]

The checker script runner (ctf-checkerslave, though I'm not happy with the name) could be redesigned into a Python library instead of importing the checker scripts as modules. Checker scripts would then be executables importing the library, instead of the other way round.

This would be the nicer design in my opinion and eliminate some configuration overhead for import-ability of checker modules. It would also provide a nice template for implementations in other languages (see #32).