refactor(gs/xanthous): Break out inventory into a common module

Creatures are going to have an inventory too now in addition to
characters, so all the data types and lenses and stuff that define
inventory need to be broken out into a separate module so the Creature
entity can use them.

Change-Id: I83f1c70d316afaaf2e75901f9dc28f79fd2cd31f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3901
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

users/grfn/xanthous/test/Xanthous/Entities/CommonSpec.hs

@@ -0,0 +1,32 @@
+--------------------------------------------------------------------------------
+module Xanthous.Entities.CommonSpec (main, test) where
+--------------------------------------------------------------------------------
+import           Test.Prelude
+import           Data.Vector.Lens (toVectorOf)
+--------------------------------------------------------------------------------
+import           Xanthous.Entities.Common
+--------------------------------------------------------------------------------
+
+main :: IO ()
+main = defaultMain test
+
+test :: TestTree
+test = testGroup "Xanthous.Entities.CommonSpec"
+  [ testGroup "Inventory"
+    [ testProperty "items === itemsWithPosition . _2" $ \inv ->
+        inv ^.. items === inv ^.. itemsWithPosition . _2
+    , testGroup "removeItemFromPosition" $
+      let rewield w inv =
+            let (old, inv') = inv & wielded <<.~ w
+            in inv' & backpack <>~ toVectorOf (wieldedItems . wieldedItem) old
+      in [ (Backpack, \item -> backpack %~ (item ^. wieldedItem <|))
+         , (LeftHand, rewield . inLeftHand)
+         , (RightHand, rewield . inRightHand)
+         , (BothHands, rewield . review doubleHanded)
+         ] <&> \(pos, addItem) ->
+           testProperty (show pos) $ \inv item ->
+             let inv' = addItem item inv
+                 inv'' = removeItemFromPosition pos (item ^. wieldedItem) inv'
+             in inv'' ^.. items === inv ^.. items
+    ]
+  ]