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/Key.hs

@@ -27,6 +27,7 @@ import qualified Data.Text as T
 import Common
 import Utility.QuickCheck
 import Utility.Bloom
+import qualified Utility.SimpleProtocol as Proto
 
 {- A Key has a unique name, which is derived from a particular backend,
  - and may contain other optional metadata. -}
@@ -129,6 +130,10 @@ instance FromJSON Key where
 	parseJSON (String t) = maybe mempty pure $ file2key $ T.unpack t
 	parseJSON _ = mempty
 
+instance Proto.Serializable Key where
+	serialize = key2file
+	deserialize = file2key
+
 instance Arbitrary Key where
 	arbitrary = Key
 		<$> (listOf1 $ elements $ ['A'..'Z'] ++ ['a'..'z'] ++ ['0'..'9'] ++ "-_\r\n \t")