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/Data/EntityMapSpec.hs

@@ -0,0 +1,26 @@
+{-# LANGUAGE ApplicativeDo #-}
+--------------------------------------------------------------------------------
+module Xanthous.Data.EntityMapSpec where
+--------------------------------------------------------------------------------
+import Test.Prelude
+--------------------------------------------------------------------------------
+import Xanthous.Data.EntityMap
+--------------------------------------------------------------------------------
+
+main :: IO ()
+main = defaultMain test
+
+test :: TestTree
+test = testGroup "Xanthous.Data.EntityMap"
+  [ testBatch $ monoid @(EntityMap Int) mempty
+  , testGroup "Eq laws"
+    [ testProperty "reflexivity" $ \(em :: EntityMap Int) ->
+        em == em
+    , testProperty "symmetric" $ \(em₁ :: EntityMap Int) em₂ ->
+        (em₁ == em₂) == (em₂ == em₁)
+    , testProperty "transitive" $ \(em₁ :: EntityMap Int) em₂ em₃ ->
+        if (em₁ == em₂ && em₂ == em₃)
+        then (em₁ == em₃)
+        else True
+    ]
+  ]