[MERGE] Add simple Storage object

[Martin Pool]

I think the idea of cleaning up Repository is a good idea, but I'm
inclined towards the path of cleaning up or deprecating the APIs
first, rather than introducing this layer.

It could be good to clearly mark the methods you are meant to override
vs those you are not.  I'd support do_xxx for those, or conversely
xxx_impl for implementation code you might want to call from
subclasses.  Having said that, because it's very open how you can
override or extend Repository so I don't think we can make an up-front
distinction between them.

Robert suggests doing these changes one method at a time, which I
think is a fine idea, as long as we previously have the conversation
about which direction they're moving in.

-- 
Martin