Reduce modifications to global lua metatables

[daurnimator]

Work towards #3

[PaulBernier]

Hi daurnimator, I wanted to thank you for your contribution and the recent issues you opened. I'm currently simply doing minimal maintenance for castl and fixing trivial bugs. I'm afraid I won't have time to review such a fundamental change in castl as it would require me to deep dive in the guts of castl and remember the ins and outs... I'm leaving this PR open for now as some people could be interested in using directly your fork (good thing you crossed referenced the issue).