KORE milestone 1

**please review**

 - unit-tested convertors from KORE to KIL and back
 - significant changes and simplifications to KORE data structures. They are smaller and more powerful now

This is in the spirit of https://github.com/kframework/k/wiki/KAST-and-KORE, where each "cell type" becomes a separate production. Hence, it will have a distinct klabel. 

I have no strong preference either, and we can change representation easily.