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

src/Xanthous/Entities/SomeEntity.hs

@@ -0,0 +1,34 @@
+{-# LANGUAGE GADTs #-}
+module Xanthous.Entities.SomeEntity
+  ( SomeEntity(..)
+  , downcastEntity
+  ) where
+
+import Xanthous.Prelude
+import Test.QuickCheck (Arbitrary(..))
+import qualified Test.QuickCheck.Gen as Gen
+
+import Xanthous.Entities (Draw(..), Entity)
+import Data.Typeable
+import Xanthous.Entities.Character
+
+data SomeEntity where
+  SomeEntity :: forall a. (Entity a, Typeable a) => a -> SomeEntity
+
+instance Show SomeEntity where
+  show (SomeEntity x) = "SomeEntity (" <> show x <> ")"
+
+instance Eq SomeEntity where
+  (SomeEntity (a :: ea)) == (SomeEntity (b :: eb)) = case eqT @ea @eb of
+    Just Refl -> a == b
+    _ -> False
+
+instance Arbitrary SomeEntity where
+  arbitrary = Gen.oneof
+    [pure $ SomeEntity Character]
+
+instance Draw SomeEntity where
+  draw (SomeEntity ent) = draw ent
+
+downcastEntity :: (Entity a, Typeable a) => SomeEntity -> Maybe a
+downcastEntity (SomeEntity e) = cast e