Refactor "config" addon to more appropriate name/locations for various classes.

Due to lack of any other ideas, we named the part which runs the rules and provides the API and Impl for them "config". That is unintuitive and confusing - that would be like calling FreeMarker templates renderer a config; or calling a SQL executor a config; or calling Groovy interpreter a config; those all are not configurations, these are rather engines.

Hence I propose to change the naming from "config" to "engine" where "config" doesn't refer to actual configuration, i.e. parameters changing behavior, not actual implementation of behavior.