Overhaul images and tags implementation

[mgax]

The images/tags code is a bit of a mess. Tags were initially supposed to work transparently, hence were implemented as symlinks, but that's actually not helpful, because a VM should always point to the same immutable base image from which it was created, so the link is dereferenced anyway. Some things that could use fixing:

• Keep images and tags in separate directories. Tags should be regular files that contain the image's ID.
• Images should have an id rather than a name; the expectation for a name is that it's human-friendly, but they are sha256 hashes.
• Validate inputs (Tag images command; improve untag command #11 (comment)).
• Allow the user to specify any unique prefix of an image id, so that it's possible to copy/paste the output of miv images and miv build into a new command.