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.

test/Xanthous/GameSpec.hs

@@ -2,10 +2,10 @@ module Xanthous.GameSpec where
 
 import Test.Prelude hiding (Down)
 import Xanthous.Game
+import Xanthous.Game.State
 import Control.Lens.Properties
 import Xanthous.Data (move, Direction(Down))
 import Xanthous.Data.EntityMap (atPosition)
-import Xanthous.Entities (SomeEntity(SomeEntity))
 
 main :: IO ()
 main = defaultMain test