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/Util.hs

@@ -26,6 +26,7 @@ module Xanthous.Util
   , takeWhileInclusive
   , smallestNotIn
   , removeVectorIndex
+  , removeFirst
   , maximum1
   , minimum1
 
@@ -49,6 +50,7 @@ import qualified Data.Vector as V
 import           Data.Semigroup (Max(..), Min(..))
 import           Data.Semigroup.Foldable
 import           Control.Monad.State.Class
+import           Control.Monad.State (evalState)
 --------------------------------------------------------------------------------
 
 newtype EqEqProp a = EqEqProp a
@@ -229,6 +231,16 @@ removeVectorIndex idx vect =
   let (before, after) = V.splitAt idx vect
   in before <> fromMaybe Empty (tailMay after)
 
+-- | Remove the first element in a sequence that matches a given predicate
+removeFirst :: IsSequence seq => (Element seq -> Bool) -> seq -> seq
+removeFirst p
+  = flip evalState False
+  . filterM (\x -> do
+                found <- get
+                let matches = p x
+                when matches $ put True
+                pure $ found || not matches)
+
 maximum1 :: (Ord a, Foldable1 f) => f a -> a
 maximum1 = getMax . foldMap1 Max