Flow slice representation

Pull useless things out of the final flows slices. Keep them only in the proto slice as it is being produced.

Share the tags of the flows, both because they are largish and we don't want to waste the space as they stay the same during the lifetime of the flow and because we may, in theory, come up with more information later on and supply it into them all.

Closes #34 (closed).

The target is set so the right diff appears, but it is expected the target branch gets to master first.