feat(xanthous): Describe *where* the item is in the inventory

When describing items in the inventory, both in detail and when
producing menus from those items, describe not just the item itself but
also *where* in the inventory the item is (either in the backpack, or
wielded in either or both of the hands). This uses a new
InventoryPosition datatype, and a method to get a list of items in the
inventory associated with their inventory position. When *removing*
items from the inventory (to wield, drop, or eat them), we want to make
sure we remove from the right position, so this also introduces
a `removeItemAtPosition` method to make that happen correctly.

Finally, some of the tests for this stuff was getting really slow - I
narrowed this down to runaway arbitrary generation for message
Templates, so I've tweaked the Arbitrary instance for that type to
generate smaller values.

Change-Id: I24e9948adae24b0ca9bf13955602108ca9079dcc
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3228
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

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

@@ -24,7 +24,6 @@ import           Data.Aeson.Generic.DerivingVia
 import           Data.FileEmbed
 import           Data.List.NonEmpty
 import           Test.QuickCheck hiding (choose)
-import           Test.QuickCheck.Arbitrary.Generic
 import           Test.QuickCheck.Instances.UnorderedContainers ()
 import           Text.Mustache
 import qualified Data.Yaml as Yaml
@@ -41,7 +40,10 @@ data Message = Single Template | Choice (NonEmpty Template)
            Message
 
 instance Arbitrary Message where
-  arbitrary = genericArbitrary
+  arbitrary =
+    frequency [ (10, Single <$> arbitrary)
+              , (1, Choice <$> arbitrary)
+              ]
   shrink = genericShrink
 
 resolve :: MonadRandom m => Message -> m Template