Add doors and an open command

Add a Door entity and an Open command, which necessitated supporting the
direction prompt. Currently nothing actually puts doors on the map,
which puts a slight damper on actually testing this out.

src/Xanthous/Generators/CaveAutomata.hs

@@ -98,10 +98,10 @@ generate' params dims = do
   let steps' = params ^. steps
   when (steps' > 0)
    $ for_ [0 .. pred steps'] . const $ stepAutomata cells dims params
-  lift $ fillOuterEdgesM cells
   -- Remove all but the largest contiguous region of unfilled space
   (_: smallerRegions) <- lift $ regions @UArray . amap not <$> freeze cells
   lift $ fillAllM (fold smallerRegions) cells
+  lift $ fillOuterEdgesM cells
   pure cells
 
 stepAutomata :: forall s g. MCells s -> Dimensions -> Params -> CellM g s ()