(PUP-2860) optimizations and maintenance

This PR contains a series of refactoring, maintenance and small performance enhancements.

The biggest source line changes are caused by a refactoring of the rgen based models splitting
them into a meta model part, and an implementation part. This to later enable faster / different construction
of the meta model and mix in the implementation.

Review is probably best done per commit.
That is deliberate, the 0.6.6 works just fine (for what we currently use), so if that is what people have installed already they can use it. But OTOH, does not harm to tell people to install the latest.