Add support for creating a composefs from a directory

[allisonkarlitskaya]

This is basically the default mode of operation for mkcomposefs, but we're interested in getting exactly the same image as if we had created it from the layer tarballs.

This requires a bit more work, including adding selinux labels and some very careful thinking about the root directory.

This introduces a new examples/unified which builds the container in a single podman build. It gets the same result inside and outside (or at least it does for me) and boots up normally.

A preparatory commit defines the inlining constant to 64 and uses it, which means that repositories need to be recreated.

Closes #34 Closes #26

[allisonkarlitskaya]

I have a bunch of pending changes for this based on the review comments, but they're better dealt with as follow-ups.