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/Add.hs

@@ -150,7 +150,7 @@ perform file = do
 
 cleanup :: Key -> Bool -> CommandCleanup
 cleanup key hascontent = do
-	maybeShowJSON $ JSONChunk [("key", key2file key)]
+	maybeShowJSON $ JSONChunk [("key", serializeKey key)]
 	when hascontent $
 		logStatus key InfoPresent
 	return True