feat(gs/xanthous): Allow throwing rocks

Implement a first pass at a "fire" command, which allows throwing rocks,
the max distance and the damage of which is based on the weight of the
item and the strength of the player.

Currently the actual numbers here likely need some tweaking, as the
rocks are easily throwable at good distances but don't really deal any
damage.

Change-Id: Ic6ad0599444af44d8438b834237a1997b67f220f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3764
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

users/grfn/xanthous/src/Xanthous/Data/EntityMap.hs

@@ -20,6 +20,7 @@ module Xanthous.Data.EntityMap
   , positions
   , lookup
   , lookupWithPosition
+  , positionOf
   -- , positionedEntities
   , neighbors
   , Deduplicate(..)
@@ -37,7 +38,7 @@ import Xanthous.Data
   , Positioned(..)
   , positioned
   , Neighbors(..)
-  , neighborPositions
+  , neighborPositions, position
   )
 import Xanthous.Data.VectorBag
 import Xanthous.Orphans ()
@@ -268,5 +269,9 @@ lookup eid = fmap (view positioned) . lookupWithPosition eid
 neighbors :: (Ord a, Show a) => Position -> EntityMap a -> Neighbors (VectorBag a)
 neighbors pos em = (\p -> view (atPosition p) em) <$> neighborPositions pos
 
+-- | Traversal to the position of the entity with the given ID
+positionOf :: EntityID -> Traversal' (EntityMap a) Position
+positionOf eid = ix eid . position
+
 --------------------------------------------------------------------------------
 makeWrapped ''Deduplicate