implementation of peer-to-peer protocol

For use with tor hidden services, and perhaps other transports later.

Based on Utility.SimpleProtocol, it's a line-based protocol,
interspersed with transfers of bytestrings of a specified size.

Implementation of the local and remote sides of the protocol is done
using a free monad. This lets monadic code be included here, without
tying it to any particular way to get bytes peer-to-peer.

This adds a dependency on the haskell package "free", although that
was probably pulled in transitively from other dependencies already.

This commit was sponsored by Jeff Goeke-Smith on Patreon.

Types/UUID.hs

@@ -13,6 +13,8 @@ import qualified Data.Map as M
 import qualified Data.UUID as U
 import Data.Maybe
 
+import qualified Utility.SimpleProtocol as Proto
+
 -- A UUID is either an arbitrary opaque string, or UUID info may be missing.
 data UUID = NoUUID | UUID String
 	deriving (Eq, Ord, Show, Read)
@@ -35,3 +37,7 @@ isUUID :: String -> Bool
 isUUID = isJust . U.fromString
 
 type UUIDMap = M.Map UUID String
+
+instance Proto.Serializable UUID where
+	serialize = fromUUID
+	deserialize = Just . toUUID