feat(xanthous) Generate random volume+density for items

Generate random volumes and densities for items based on the ranges for
those two quantities in the raw when building instances of items.

Since this is the first time creating an item is impure, this also lifts
entity generation into a (random) monadic context

Change-Id: I2de4880e8144f7ff9e1304eb32806ed1d7affa18
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3226
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

users/grfn/xanthous/test/Xanthous/Game/StateSpec.hs

@@ -5,6 +5,8 @@ import           Test.Prelude
 --------------------------------------------------------------------------------
 import           Xanthous.Game.State
 import           Xanthous.Entities.Raws (raws, entityFromRaw)
+import Control.Monad.Random (evalRandT)
+import System.Random (getStdGen)
 --------------------------------------------------------------------------------
 
 main :: IO ()
@@ -13,13 +15,15 @@ main = defaultMain test
 test :: TestTree
 test = testGroup "Xanthous.Game.StateSpec"
   [ testGroup "entityTypeName"
-    [ testCase "for a creature" $
+    [ testCase "for a creature" $ do
         let gormlakRaw = raws ^?! ix "gormlak"
-            creature = entityFromRaw gormlakRaw
-        in entityTypeName creature @?= "Creature"
-    , testCase "for an item" $
+        creature <- runRand $ entityFromRaw gormlakRaw
+        entityTypeName creature @?= "Creature"
+    , testCase "for an item" $ do
         let stickRaw = raws ^?! ix "stick"
-            item = entityFromRaw stickRaw
-        in entityTypeName item @?= "Item"
+        item <- runRand $ entityFromRaw stickRaw
+        entityTypeName item @?= "Item"
     ]
   ]
+  where
+    runRand x = evalRandT x =<< getStdGen