accdb: add a convenience function for searching
accdb: port 'tag' to search filter
accdb: get rid of 'PrivateAttribute' types

This can be done in Entry.dump(); in fact, it *is* already done in
Entry.dump(), only with the condition reversed, so there's no actual
advantage in imposing this on other code.
accdb: hide debug messages
accdb: add 'copy' and 'move' changeset ops
accdb: deduplicate a bunch of identical changeset code
accdb: stop grouping changesets by key

This fixes unexpected (and inconsistent due to dict randomization)
behavior when one key's changes reference another (copy/move values),
e.g. ["foo<=bar", "-bar"] would be applied in unpredictable order.