Add support for passing effective statement dependencies

YANGTOOLS-1197 and YANGTOOLS-1198 are all about storing a proper reference which is picked up by createEffective(Current) via namespaces.

There are quite a few places which perform this kind of trick – having a lot of namespaces for essentially the same purpose.

Consider extending EffectiveStmtCtx.Current with a key/value lookup mechanism, populated by (for example) StmtContext.Mutable.setEffectiveParameter(key, value).

This would provide a well-defined blueprint how to solve this kind of a problem without open-coding it all over the place.