rename key2file and file2key

What these generate is not really suitable to be used as a filename,
which is why keyFile and fileKey further escape it. These are just
serializing Keys.

Also removed a quickcheck test that was very unlikely to test anything
useful, since it relied on random chance creating something that looks
like a serialized key. The other test is sufficient for testing what
that was intended to test anyway.

Assistant/Unused.hs

@@ -74,7 +74,7 @@ expireUnused duration = do
 	now <- liftIO getPOSIXTime
 	let oldkeys = M.keys $ M.filter (tooold now) m
 	forM_ oldkeys $ \k -> do
-		debug ["removing old unused key", key2file k]
+		debug ["removing old unused key", serializeKey k]
 		liftAnnex $ tryNonAsync $ do
 			lockContentForRemoval k removeAnnex
 			logStatus k InfoMissing