TransformationCombinators: Start cleaning up
Continue experiments
Save IDEA workspace
Renames
Make interface to TransformerCombinators more typed

I'm not so happy with this, I'll keep experimenting.
Use AbstractPartialFunction better

And move tests.
Remove stupid experiment

The approach I remove would have had worse performance and offers
no advantage over the one I came up with - the new one just took
slightly longer to figure out.
Make import local
Improve performance of transformer combinators

Also, add tracing code to observe their behavior.
Prototype generalized intersection/union operator for bags

Rewriting `bag1 filter (bag2 contains)` as standard intersection is only
possible for sets, not for bags. This generalized operator supports that.