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/test/Xanthous/UtilSpec.hs

@@ -34,4 +34,10 @@ test = testGroup "Xanthous.Util"
     [ testCase "_1 += 1"
       $ execState (modifyKL _1 $ pure . succ) (1 :: Int, 2 :: Int) @?= (2, 2)
     ]
+  , testGroup "removeFirst"
+    [ testCase "example" $
+      removeFirst @[Int] (> 5) [1..10] @?= [1, 2, 3, 4, 5, 7, 8, 9, 10]
+    , testProperty "the result is the right length" $ \(xs :: [Int]) p ->
+        length (removeFirst p xs) `elem` [length xs, length xs - 1]
+    ]
   ]