Add first-class support for Report on Change

[bradh]

Task Background
This is described in ST 0107, and is invoked by a few standards (e.g. ST 0601, MIMD).

Probably a breaking change, which would be good to fix for 2.x.

Completion criteria
[ ] Evaluate what the design would look like for both reading and writing
[ ] Add interfaces and common code, plus unit tests
[ ] Update all user code
[ ] Add and/or update example(s) demonstrating the functionality
[ ] Revise documentation

Implementation Notes and Ideas
The key concept is the zero length item (ZLI).
It feels like there should be a context layer that keeps history and tracks age for each item.
The Viewer kind of does this as a side effect of the way in which the tree updates. That could be modified to show age of the entry (e.g. green for last update, orange for aging, red for out-of-date).
There is possibly a way to properly support repeating items within a single local set using this.