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.

Annex/VariantFile.hs

@@ -34,8 +34,8 @@ mkVariant file variant = takeDirectory file
  -}
 variantFile :: FilePath -> Key -> FilePath
 variantFile file key
-	| doubleconflict = mkVariant file (key2file key)
-	| otherwise = mkVariant file (shortHash $ key2file key)
+	| doubleconflict = mkVariant file (serializeKey key)
+	| otherwise = mkVariant file (shortHash $ serializeKey key)
   where
 	doubleconflict = variantMarker `isInfixOf` file