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.

src/Xanthous/Entities/Character.hs

@@ -0,0 +1,21 @@
+module Xanthous.Entities.Character
+  ( Character(..)
+  , mkCharacter
+  ) where
+
+import Xanthous.Prelude
+import Test.QuickCheck
+
+import Xanthous.Entities
+
+data Character where
+  Character :: Character
+  deriving stock (Show, Eq, Ord, Generic)
+  deriving anyclass (CoArbitrary, Function)
+  deriving Draw via (DrawCharacter "@" Character)
+
+instance Arbitrary Character where
+  arbitrary = pure Character
+
+mkCharacter :: Character
+mkCharacter = Character