Redesigned `digest` cmd

This new digest cmd compiles, fingerprints, and outputs source
files.  You can run it via the cli with `dassets digest [PATHS] or
programmatically with `Dassets.digest(paths)`.

@jcredding - a lot of "churn" in the test suite on this one, but essentially swapped out the old n busted digest cmd implementation for this new one that does all the things.  Ready for review.