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.

RemoteDaemon/Types.hs

@@ -100,10 +100,6 @@ instance Proto.Serializable RemoteURI where
 	serialize (RemoteURI u) = show u
 	deserialize = RemoteURI <$$> parseURI
 
-instance Proto.Serializable [Char] where
-	serialize = id
-	deserialize = Just
-
 instance Proto.Serializable RefList where
 	serialize = unwords . map Git.fromRef
 	deserialize = Just . map Git.Ref . words