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.

Command/AddUrl.hs

@@ -400,7 +400,7 @@ addWorkTree u url file key mtmp = case mtmp of
 			else void $ Command.Add.addSmall file
   where
 	go = do
-		maybeShowJSON $ JSONChunk [("key", key2file key)]
+		maybeShowJSON $ JSONChunk [("key", serializeKey key)]
 		setUrlPresent key url
 		logChange key u InfoPresent
 		ifM (addAnnexedFile file key mtmp)