Add .scan() method for databanks

A method to get all items of a type. Useful for e.g. backing up a
databank. Also for generating roll-up data (count) if you haven't been
storing it before.

Most drivers already implement something like it internally
for search without an index, so it seemed safe to surface it.
Document the scan() method.
Update the version number since we have a new API
Add Cls.scan() to Databank object classes
Document the scan() class method