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/Spec.hs

@@ -1,3 +1,14 @@
--- |
+import Test.Prelude
+import qualified Xanthous.DataSpec
+import qualified Xanthous.Data.EntityMapSpec
+import qualified Xanthous.GameSpec
 
-module Spec where
+main :: IO ()
+main = defaultMain test
+
+test :: TestTree
+test = testGroup "Xanthous"
+  [ Xanthous.DataSpec.test
+  , Xanthous.Data.EntityMapSpec.test
+  , Xanthous.GameSpec.test
+  ]