Obnam: representing chunk IDs, part 2

Goal Last time I started changing how chunk IDs are represented. Today I'll finish that work. Plan Change all code to use new Id and drop the old Id . Notes Reviewing what I did last time, I see I have no doc test that serves as an example for the new Id and IdFactory . That's easy to rectify. Next, the goal is to use the new stuff instead of the old stuff. Generating the old Id was infallible.…