Add entities, and allow walking around

Add support for entities via a port of the EntityMap type, and implement
command support starting at basic hjkl.

test/Xanthous/GameSpec.hs

@@ -0,0 +1,30 @@
+module Xanthous.GameSpec where
+
+import Test.Prelude hiding (Down)
+import Xanthous.Game
+import Control.Lens.Properties
+import Xanthous.Data (move, Direction(Down))
+import Xanthous.Data.EntityMap (atPosition)
+import Xanthous.Entities.SomeEntity
+
+main :: IO ()
+main = defaultMain test
+
+test :: TestTree
+test = testGroup "Xanthous.Game"
+  [ testGroup "positionedCharacter"
+    [ testProperty "lens laws" $ isLens positionedCharacter
+    , testCase "updates the position of the character" $ do
+      let initialGame = getInitialState
+          initialPos = initialGame ^. characterPosition
+          updatedGame = initialGame & characterPosition %~ move Down
+          updatedPos = updatedGame ^. characterPosition
+      updatedPos @?= move Down initialPos
+      updatedGame ^. entities . atPosition initialPos @?= fromList []
+      updatedGame ^. entities . atPosition updatedPos
+        @?= fromList [SomeEntity $ initialGame ^. character]
+    ]
+  , testGroup "characterPosition"
+    [ testProperty "lens laws" $ isLens characterPosition
+    ]
+  ]