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.

Remote/Helper/Ssh.hs

@@ -67,7 +67,7 @@ git_annex_shell cs r command params fields
 			else params
 		return (Param command : File dir : params')
 	uuidcheck NoUUID = []
-	uuidcheck (UUID u) = ["--uuid", u]
+	uuidcheck u@(UUID _) = ["--uuid", fromUUID u]
 	fieldopts
 		| null fields = []
 		| otherwise = fieldsep : map fieldopt fields ++ [fieldsep]