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.

git-annex.cabal

@@ -342,6 +342,7 @@ Executable git-annex
    MissingH,
    hslogger,
    monad-logger,
+   free,
    utf8-string,
    bytestring,
    text,
@@ -918,6 +919,7 @@ Executable git-annex
     Remote.Helper.Hooks
     Remote.Helper.Http
     Remote.Helper.Messages
+    Remote.Helper.P2P
     Remote.Helper.ReadOnly
     Remote.Helper.Special
     Remote.Helper.Ssh