refactor(gs/xanthous): Use a Word for the level number

Using a signed Int here is a little silly, since we can never have
negative levels.

Change-Id: Ibe03be5014226e07dfa6f78d8360301bc1b7c9b1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3803
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

users/grfn/xanthous/src/Xanthous/App.hs

@@ -566,7 +566,7 @@ showPanel panel = do
 --------------------------------------------------------------------------------
 
 genLevel
-  :: Int -- ^ level number
+  :: Word -- ^ Level number, starting at 0
   -> AppM Level
 genLevel _num = do
   let dims = Dimensions 80 80