* keys.new-keeper now writes its nubs into $SAFE rather than the
caller's current directory.
* keys.reveal and keys.stash insist on reading their input from stdin
rather than a file name.
* keys.keeper-cards writes its output to stdout, and collects input
nubs from $SAFE.
* keys.keeper-nub is a new tool which extracts a keeper nub on demand.
Some of the tools have also had their error messages improved.