nbc-change-stmt

[abierman]

There is a need for a new statement to identify an NBC change in an exportable statement.
The Semver extension is not helpful since it is at the module-level instead of the object level,

This cannot be done with the 'deprecated' statement in #130 since the NBC change is most likely
done in a 'current' definition with no intent to deprecate and replace it.

Not suggesting a detailed solution, but here is an outline:

revision 2020-02-10 {
  rev:non-backwards-compatible;
}

container mycon {
   rev:nbc-change 2020-02-10;
   ...
}