Refactor module structure for runtime modules

[jberthold]

Maybe we can just move code to different modules when we modify or extend it.

There was already a proposal to refactor the modules,. adding an umbrella runtime.md file and module.
Having an overall RUNTIME module is certainly a good thing, too. In which case the code from kmir.md that handles statements and terminators should move to its own module as well, and kmir.md only imports other things and defines the very top (#init, which we don't even use).

Originally posted by @jberthold in #674 (comment)