Aha! p/v.archive will be great new Leonine resources

[Edward K. Ream]

Aha! Leo should define p/v.archive methods. These will be fundamental new resources for Leo.

This Aha arose while considering difficult undo/redo questions. But its significance is more general. These methods:

- Might replace hacks throughout Leo.

- Form a bridge between the Position and VNode worlds.

The unified PR contains the first draft of these methods.

Edward