Add items and inventory

Add a new "Item" entity, which pulls from the previously-existent
ItemType raw, and add a "PickUp" command which takes the (currently
*only*) item off the ground and puts it into the inventory.

src/Xanthous/Entities/Item.hs

@@ -0,0 +1,35 @@
+{-# LANGUAGE TemplateHaskell #-}
+module Xanthous.Entities.Item
+  ( Item(..)
+  , itemType
+  , newWithType
+  ) where
+--------------------------------------------------------------------------------
+import Xanthous.Prelude
+import Test.QuickCheck
+import Data.Aeson (ToJSON, FromJSON)
+import Data.Aeson.Generic.DerivingVia
+--------------------------------------------------------------------------------
+import Xanthous.Entities.RawTypes hiding (Item)
+import Xanthous.Entities (Draw(..), Entity(..), DrawRawChar(..))
+--------------------------------------------------------------------------------
+
+data Item = Item
+  { _itemType :: ItemType
+  }
+  deriving stock (Eq, Show, Generic)
+  deriving anyclass (CoArbitrary, Function)
+  deriving Draw via DrawRawChar "_itemType" Item
+  deriving (ToJSON, FromJSON)
+       via WithOptions '[ FieldLabelModifier '[Drop 1] ]
+                       Item
+makeLenses ''Item
+
+instance Arbitrary Item where
+  arbitrary = Item <$> arbitrary
+
+instance Entity Item where
+  blocksVision _ = False
+
+newWithType :: ItemType -> Item
+newWithType = Item