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/test/Spec.hs

@@ -6,6 +6,7 @@ import qualified Xanthous.Data.EntityCharSpec
 import qualified Xanthous.Data.EntityMap.GraphicsSpec
 import qualified Xanthous.Data.EntityMapSpec
 import qualified Xanthous.Data.LevelsSpec
+import qualified Xanthous.Data.MemoSpec
 import qualified Xanthous.Data.NestedMapSpec
 import qualified Xanthous.DataSpec
 import qualified Xanthous.Entities.RawsSpec
@@ -30,6 +31,7 @@ test = testGroup "Xanthous"
   , Xanthous.Data.EntityMap.GraphicsSpec.test
   , Xanthous.Data.EntityMapSpec.test
   , Xanthous.Data.LevelsSpec.test
+  , Xanthous.Data.MemoSpec.test
   , Xanthous.Data.NestedMapSpec.test
   , Xanthous.DataSpec.test
   , Xanthous.Entities.RawsSpec.test