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/Map.hs

@@ -113,7 +113,7 @@ nodeId :: Git.Repo -> String
 nodeId r =
 	case getUncachedUUID r of
 		NoUUID -> Git.repoLocation r
-		UUID u -> u
+		u@(UUID _) -> fromUUID u
 
 {- A node representing a repo. -}
 node :: M.Map UUID String -> [Git.Repo] -> TrustMap -> RepoRemotes -> String