Add support for multiple levels

Add a data structure, based on the zipper comonad, which provides
support for multiple levels, each of which is its own entity map. The
current level is provided by coreturn, which the `entities` lens has
been updated to use. Nothing currently supports going up or down levels
yet - that's coming next.

test/Xanthous/Data/LevelsSpec.hs

@@ -0,0 +1,60 @@
+--------------------------------------------------------------------------------
+module Xanthous.Data.LevelsSpec (main, test) where
+--------------------------------------------------------------------------------
+import Test.Prelude hiding (levels)
+--------------------------------------------------------------------------------
+import qualified Data.Aeson as JSON
+--------------------------------------------------------------------------------
+import Xanthous.Util (between)
+import Xanthous.Data.Levels
+--------------------------------------------------------------------------------
+
+main :: IO ()
+main = defaultMain test
+
+test :: TestTree
+test = testGroup "Xanthous.Data.Levels"
+  [ testGroup "current"
+    [ testProperty "view is extract" $ \(levels :: Levels Int) ->
+        levels ^. current === extract levels
+    , testProperty "set replaces current" $ \(levels :: Levels Int) new ->
+        extract (set current new levels) === new
+    , testProperty "set extract is id" $ \(levels :: Levels Int) ->
+        set current (extract levels) levels === levels
+    , testProperty "set y ∘ set x ≡ set y" $ \(levels :: Levels Int) x y ->
+        set current y (set current x levels) === set current y levels
+    ]
+  , localOption (QuickCheckTests 20)
+  $ testBatch $ semigroup @(Levels Int) (error "unused", 1 :: Int)
+  , testGroup "next/prev"
+    [ testGroup "nextLevel"
+      [ testProperty "seeks forwards" $ \(levels :: Levels Int) genned ->
+          (pos . runIdentity . nextLevel (Identity genned) $ levels)
+          === pos levels + 1
+      , testProperty "maintains the invariant" $ \(levels :: Levels Int) genned ->
+          let levels' = runIdentity . nextLevel (Identity genned) $ levels
+          in between 0 (length levels') $ pos levels'
+      , testProperty "extract is total" $ \(levels :: Levels Int) genned ->
+          let levels' = runIdentity . nextLevel (Identity genned) $ levels
+          in total $ extract levels'
+      ]
+    , testGroup "prevLevel"
+      [ testProperty "seeks backwards" $ \(levels :: Levels Int) ->
+          case prevLevel levels of
+            Nothing -> property Discard
+            Just levels' -> pos levels' === pos levels - 1
+      , testProperty "maintains the invariant" $ \(levels :: Levels Int) ->
+          case prevLevel levels of
+            Nothing -> property Discard
+            Just levels' -> property $ between 0 (length levels') $ pos levels'
+      , testProperty "extract is total" $ \(levels :: Levels Int) ->
+          case prevLevel levels of
+            Nothing -> property Discard
+            Just levels' -> total $ extract levels'
+      ]
+    ]
+  , testGroup "JSON"
+    [ testProperty "toJSON/parseJSON round-trip" $ \(levels :: Levels Int) ->
+        JSON.decode (JSON.encode levels) === Just levels
+    ]
+  ]