convert UUID from String to ByteString

This should make == comparison of UUIDs somewhat faster, and perhaps a
few other operations around maps of UUIDs etc.

FromUUID/ToUUID are used to convert String, which is still used for all
IO of UUIDs. Eventually the hope is those instances can be removed,
and all git-annex branch log files etc use ByteString throughout, for a
real speed improvement.

Note the use of fromRawFilePath / toRawFilePath -- while a UUID usually
contains only alphanumerics and so could be treated as ascii, it's
conceivable that some git-annex repository has been initialized using
a UUID that is not only not a canonical UUID, but contains high unicode
or invalid unicode. Using the filesystem encoding avoids any problems
with such a thing. However, a NUL in a UUID seems extremely unlikely,
so I didn't use encodeBS / decodeBS to avoid their extra overhead in
handling NULs.

The Read/Show instance for UUID luckily serializes the same way for
ByteString as it did for String.

Command/TransferKeys.hs

@@ -82,7 +82,7 @@ sendRequest :: Transfer -> TransferInfo -> Handle -> IO ()
 sendRequest t tinfo h = do
 	hPutStr h $ intercalate fieldSep
 		[ serialize (transferDirection t)
-		, maybe (serialize (fromUUID (transferUUID t)))
+		, maybe (serialize ((fromUUID (transferUUID t)) :: String))
 			(serialize . Remote.name)
 			(transferRemote tinfo)
 		, serialize (transferKey t)