[server] Add ForkModule for combining other modules

This one has some interesting code in its constructor, using the Cont monad
because of the way the ModuleConstructor is structured.

src/Forest/TreeModule.hs

@@ -8,9 +8,16 @@ import qualified Data.Text   as T
 import           Forest.Node
 
 class TreeModule a where
-  edit   :: a -> Path -> T.Text -> IO ()
+  edit :: a -> Path -> T.Text -> IO ()
+  edit _ _ _ = pure ()
+
   delete :: a -> Path -> IO ()
-  reply  :: a -> Path -> T.Text -> IO ()
-  act    :: a -> Path -> IO ()
+  delete _ _ = pure ()
+
+  reply :: a -> Path -> T.Text -> IO ()
+  reply _ _ _ = pure ()
+
+  act :: a -> Path -> IO ()
+  act _ _ = pure ()
 
 type ModuleConstructor a = (Node -> IO ()) -> (a -> IO ()) -> IO ()