rename digests_file to digests

Shorter name.  Not so concerned with the fact that this is backed
by a file - more importantly it just models the digested files, or
"digests".
cleanups to digests modeling

Removed some unneeded methods and tried to simplify the api.  This
is prepping for the new compile behavior.  The digests stores the
previous compile state and is needed for pruning old compiled files
from the output on compile.

This is, of course, in addition to its asset file handling for the
middleware.