feat(gs/xanthous): Creatures drop their items when they die

When a creature is killed, it now drops all the contents of its
inventory on the ground.

Change-Id: Ie95c397308ba2c9861f10e58b99c91c9cc759c56

users/grfn/xanthous/src/Xanthous/Util.hs

@@ -34,7 +34,7 @@ module Xanthous.Util
   , times, times_, endoTimes
 
     -- * State utilities
-  , modifyK, modifyKL
+  , modifyK, modifyKL, useListOf
 
     -- * Type-level programming utils
   , KnownBool(..)
@@ -311,6 +311,14 @@ modifyK k = get >>= k >>= put
 modifyKL :: MonadState s m => LensLike m s s a b -> (a -> m b) -> m ()
 modifyKL l k = get >>= traverseOf l k >>= put
 
+-- | Use a list of all the targets of a 'Fold' in the current state
+--
+-- @@
+-- evalState (useListOf folded) === toList
+-- @@
+useListOf :: MonadState s m => Getting (Endo [a]) s a -> m [a]
+useListOf = gets . toListOf
+
 --------------------------------------------------------------------------------
 
 -- | A newtype wrapper around 'Char' whose 'Enum' and 'Bounded' instances only