add a UUID type

Should have done this a long time ago.

Types/UUID.hs

@@ -7,5 +7,15 @@
 
 module Types.UUID where
 
--- might be nice to have a newtype, but lots of stuff treats uuids as strings
-type UUID = String
+-- A UUID is either an arbitrary opaque string, or UUID info may be missing.
+data UUID = NoUUID | UUID String
+	deriving (Eq, Ord)
+
+instance Show UUID where
+	show (UUID u) = u
+	show NoUUID = ""
+
+instance Read UUID where
+	readsPrec _ s
+		| null s = [(NoUUID, "")]
+		| otherwise = [(UUID s, "")]