Switch to a simpler progress bar library. I've been having trouble tracking
down a panic that seemed to affect the prior code; the new code is much
easier and uses far fewer async channels and things.
Since packing can be quite slow, it is nice to display progress to the
caller. We use the excellent github.com/vbauerster/mpb library to do so,
and add a bit of colour with github.com/logrusru/aurora.
Finally, augment the inspector and packer with a summary printer that
displays the overall file size/count, and compression ratio for each
compression type.
