feat(xanthous): Memoize characterVisiblePositions

Memoize the return value of characterVisiblePositions to a new,
semi-abstracted "memo" field on the GameState, recalcuclated if the
character position ever changes. I'm 90% sure that the perf issues we
were encountering were actually caused by characterVisiblePositions
getting called once for *every tile* on draw, but this slightly larger
change also makes the game perform relatively-usably again.

Since this is only recalculated if the character position changes, if we
ever get non-transparent entities moving around without the characters
influence (maybe something building or knocking down walls?) we'll have
an issue there where the vision won't be updated as a result of those
changes if they happen while the character is taking a non-moving action
- but we can cross that bridge when we come to it.

Change-Id: I3fc745ddf0014d6f164f735ad7e5080da779b92a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3185
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

users/grfn/xanthous/src/Xanthous/App.hs

@@ -216,9 +216,7 @@ handleCommand Close = do
 
 handleCommand Look = do
   prompt_ @'PointOnMap ["look", "prompt"] Cancellable
-    $ \(PointOnMapResult pos) ->
-      gets (revealedEntitiesAtPosition pos)
-      >>= \case
+    $ \(PointOnMapResult pos) -> revealedEntitiesAtPosition pos >>= \case
         Empty -> say_ ["look", "nothing"]
         ents -> describeEntities ents
   continue