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/Orphans.hs

@@ -84,9 +84,9 @@ instance Arbitrary Pos where
   shrink (unPos -> x) = mkPos <$> [x..1]
 
 instance Arbitrary Node where
-  arbitrary = sized node
+  arbitrary = scale (`div` 10) $ sized node
     where
-      node n | n > 0 = oneof $ leaves ++ branches (n `div` 2)
+      node n | n > 0 = oneof $ leaves ++ branches (n `div` 4)
       node _ = oneof leaves
       branches n =
         [ Section <$> arbitrary <*> subnodes n
@@ -110,7 +110,7 @@ concatTextBlocks (TextBlock txt₁ : TextBlock txt₂ : xs)
 concatTextBlocks (x : xs) = x : concatTextBlocks xs
 
 instance Arbitrary Template where
-  arbitrary = do
+  arbitrary = scale (`div` 8) $ do
     template <- concatTextBlocks <$> arbitrary
     -- templateName <- arbitrary
     -- rest <- arbitrary