Found this in failed armhf build log, where quickcheck found a way to break
prop_idempotent_key_decode. The "s" indicates size, but since nothing comes
after it, that's not valid. When encoding the resulting key, no size was
present, so it encoded to "a--a".
Also, "a-sX--a" is not legal, since X is not a number. Not found by
quickcheck.
A file named "foo-" or "foo-bar" was taken as a key's file, with a backend
of "foo", and an empty keyName. This led to various problems, especially
because converting that key back to a file did not yeild the same filename.
This commit includes a paydown on technical debt incurred two years ago,
when I didn't know that it was bad to make custom Read and Show instances
for types. As the routes need Read and Show for Transfer, which includes a
Key, and deriving my own Read instance of key was not practical,
I had to finally clean that up.
So the compact Key read and show functions are now file2key and key2file,
and Read and Show are now derived instances.
Changed all code that used the old instances, compiler checked.
(There were a few places, particularly in Command.Unused, and the test
suite where the Show instance continue to be used for legitimate
comparisons; ie show key_x == show key_y (though really in a bloom filter))
