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/Import.hs

@@ -86,7 +86,7 @@ start largematcher mode (srcfile, destfile) =
 		)
   where
 	deletedup k = do
-		showNote $ "duplicate of " ++ key2file k
+		showNote $ "duplicate of " ++ serializeKey k
 		verifyExisting k destfile
 			( do
 				liftIO $ removeFile srcfile