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.

Utility/SimpleProtocol.hs

@@ -5,6 +5,9 @@
  - License: BSD-2-clause
  -}
 
+{-# LANGUAGE FlexibleInstances #-}
+{-# OPTIONS_GHC -fno-warn-orphans #-}
+
 module Utility.SimpleProtocol (
 	Sendable(..),
 	Receivable(..),
@@ -88,3 +91,7 @@ dupIoHandles = do
 	nullh `hDuplicateTo` stdin
 	stderr `hDuplicateTo` stdout
 	return (readh, writeh)
+
+instance Serializable [Char] where
+	serialize = id
+	deserialize = Just