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.

Remote/External/Types.hs

@@ -250,14 +250,6 @@ instance Proto.Serializable Direction where
 	deserialize "RETRIEVE" = Just Download
 	deserialize _ = Nothing
 
-instance Proto.Serializable Key where
-	serialize = key2file
-	deserialize = file2key
-
-instance Proto.Serializable [Char] where
-	serialize = id
-	deserialize = Just
-
 instance Proto.Serializable ProtocolVersion where
 	serialize = show
 	deserialize = readish