decode_c converted to ByteString

This speeds up a few things, notably CmdLine.Seek using Git.Filename
which uses decode_c and this avoids a conversion to String and back,
and probably the ByteString implementation of decode_c is also faster
for simple cases at least than the string version.

encode_c cannot be converted to ByteString (or if it did, it would have
to convert right back to String in order to handle unicode).

Sponsored-by: Brock Spratlen on Patreon

Git/Filename.hs

@@ -26,8 +26,7 @@ decode b = case S.uncons b of
 			Nothing -> b
 			Just (i, l)
 				| l /= q -> b
-				| otherwise ->
-					encodeBS $ decode_c $ decodeBS i
+				| otherwise -> decode_c i
   where
   	q :: Word8
 	q = fromIntegral (ord '"')