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/AddUnused.hs

@@ -32,7 +32,7 @@ perform key = next $ do
 	addLink file key Nothing
 	return True
   where
-	file = "unused." ++ key2file key
+	file = "unused." ++ serializeKey key
 
 {- The content is not in the annex, but in another directory, and
  - it seems better to error out, rather than moving bad/tmp content into