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/Xanthous/Data/MemoSpec.hs

@@ -0,0 +1,19 @@
+--------------------------------------------------------------------------------
+module Xanthous.Data.MemoSpec (main, test) where
+--------------------------------------------------------------------------------
+import Test.Prelude
+import Test.QuickCheck.Instances.Text ()
+--------------------------------------------------------------------------------
+import Xanthous.Data.Memo
+--------------------------------------------------------------------------------
+
+main :: IO ()
+main = defaultMain test
+
+test :: TestTree
+test = testGroup "Xanthous.Data.MemoSpec"
+  [ testGroup "getMemoized"
+    [ testProperty "when key matches" $ \k v ->
+        getMemoized @Int @Int k (memoizeWith k v) === Just v
+    ]
+  ]