Declarative Shadow Variables (Full Support)

[triceo]

Get "Declarative Shadow Variables" from a "Preview feature" status to a regular feature. Before that can happen:

• The feature needs to have a stable API.
• All behaviors and corner cases need to be understood and documented.
• Performance on large datasets needs to be satisfactory.

The sub-tasks exist as milestones in this work.

[kentbill]

This is an excellent feature!