Display multiple messages per turn

When tracking message history, save messages associated with the turn
they were displayed on, which allows us to have the notion of the
"current turn's" messages (provided via a MonoComonad instance).

src/Xanthous/App.hs

@@ -103,7 +103,7 @@ handleEvent ev = use promptState >>= \case
 handleNoPromptEvent :: BrickEvent Name () -> AppM (Next GameState)
 handleNoPromptEvent (VtyEvent (EvKey k mods))
   | Just command <- commandFromKey k mods
-  = do messageHistory %= hideMessage
+  = do messageHistory %= nextTurn
        handleCommand command
 handleNoPromptEvent _ = continue
 
@@ -135,7 +135,7 @@ handleCommand PickUp = do
   continue
 
 handleCommand PreviousMessage = do
-  messageHistory %= popMessage
+  messageHistory %= previousMessage
   continue
 
 handleCommand Open = do