Progressively reveal the map to the player

As the character walks around the map, progressively reveal the entities
on the map to them, using an algorithm based on well known
circle-rasterizing and line-rasterizing algorithms to calculate lines of
sight that are potentially obscured by walls.

test/Xanthous/Util/GraphicsSpec.hs

@@ -0,0 +1,39 @@
+module Xanthous.Util.GraphicsSpec (main, test) where
+--------------------------------------------------------------------------------
+import Test.Prelude hiding (head)
+--------------------------------------------------------------------------------
+import Xanthous.Util.Graphics
+import Xanthous.Util
+import Data.List (head)
+--------------------------------------------------------------------------------
+
+main :: IO ()
+main = defaultMain test
+
+test :: TestTree
+test = testGroup "Xanthous.Util.Graphics"
+  [ testGroup "circle"
+    [ testCase "radius 12, origin 0"
+      $ (sort . unique @[] @[_]) (circle @Int (0, 0) 12)
+      @?= (sort . unique) (
+        let quadrant =
+              [ (0, 12) , (1, 12) , (2, 12) , (3, 12)
+              , (4, 12) , (5, 11) , (6, 11) , (7, 10)
+              , (8, 9)  , (9, 9)  , (9, 8)  , (10, 7)
+              , (11, 6) , (11, 5) , (12, 4) , (12, 3)
+              , (12, 2) , (12, 1) , (12, 0)
+              ]
+        in  quadrant
+         <> (quadrant <&> _1 %~ negate)
+         <> (quadrant <&> _2 %~ negate)
+         <> (quadrant <&> both %~ negate)
+      )
+    ]
+
+  , testGroup "line"
+    [ testProperty "starts and ends at the start and end points" $ \start end ->
+        let ℓ = line @Int start end
+        in counterexample ("line: " <> show ℓ)
+        $ length ℓ > 2 ==> (head ℓ === start) .&&. (head (reverse ℓ) === end)
+    ]
+  ]