Dropped transcoding, added standin-file

[marenamat]

This is another take on how we actually can work with standins. After discussing with @vvilimek and reading RFC 7951 and 9254, we found out that transcoding is not resolved in these documents at all, and therefore it makes no sense to suddenly specify all the intermediate nodes and round trips here.

I have split the transcoding into a separate document, mentioning also Netconf / Restconf / Coreconf translation, as there is quite a lot to work through.

We also thought about how to distribute the answer to "which standins have been used", and realized that in addition to the main yang file, one always also needs to distribute the SID files, and therefore the channels are already there. Thus one can simply create another file, this time specifying how the values are encoded, and distribute it along. We should then add this into the Constrained YANG Library.

This pull request is mostly a request for comments and discussion. There is some more work needed and I don't wanna dump time into making things precisely if you find some major problems. Yet as for now, we think that this is a more viable path than what we were trying to do before.

(This pull request semantically supersedes #10)

[cabo]

reading RFC 7951 and 9254, we found out that transcoding is not resolved in these documents at all, and therefore it makes no sense to suddenly specify all the intermediate nodes and round trips here.

Hmm.
The terms are useful, I wouldn't want to remove them from this document.
The specification which legacy representations are canonical (e.g., case of UUID) also is best kept with the specification of the stand-in tags themselves.