Add structures for tool output

[avtobiff]

This adds data structures and also defines the list of YANG changes.

The YANG model define a structure which the on-wire algorithm uses.

The ASN.1 structure defines a structure that is used for the schema algorithm.

The list of YANG changes will define all CHANGES and how tools should
report them.

[avtobiff]

If it is not possible to define a YANG model for the data format,
perhaps a data format should be decided on instead e.g. something
encoded in JSON. This way it is easier to create tooling that
analyze the YANG diffs.