Use menus for combat and picking up items

Refactor a bunch of stuff around to allow for polymorphically surfacing
an EntityChar for all entities, and use this to write a generic
`entityMenu` function, which generates a menu from the chars of a list
of entities - and use that to fully implement (removing `undefined`)
menus for both attacking and picking things up when there are multiple
entities on the relevant tile.

src/Xanthous/Game/Draw.hs

@@ -12,7 +12,7 @@ import           Brick.Widgets.Edit
 import           Xanthous.Data
 import           Xanthous.Data.EntityMap (EntityMap, atPosition)
 import qualified Xanthous.Data.EntityMap as EntityMap
-import           Xanthous.Entities
+import           Xanthous.Game.State
 import           Xanthous.Entities.Character
 import           Xanthous.Game
                  ( GameState(..)