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/MessageSpec.hs

@@ -38,7 +38,7 @@ test = testGroup "Xanthous.Messages"
           let
             Right tpl = compileMustacheText "foo" "bar"
             msg = Single tpl
-            mm = Nested $ [("foo", Direct msg)]
+            mm = Nested [("foo", Direct msg)]
           in mm ^? ix ["foo"] @?= Just msg
         ]
     , testGroup "lookupMessage"
@@ -50,4 +50,10 @@ test = testGroup "Xanthous.Messages"
   , testGroup "Messages"
     [ testCase "are all valid" $ messages `deepseq` pure ()
     ]
+
+  , testGroup "Template"
+    [ testGroup "eq"
+      [ testProperty "reflexive" $ \(tpl :: Template) -> tpl == tpl
+      ]
+    ]
   ]