Convert generated levels to walls

Add support for converting generated levels to walls, and merge one into
the entity map at the beginning of the game.

There's nothing here that guarantees the character ends up *inside* the
level though (they almost always don't) so that'll have to be slotted
into the level generation process.

src/Xanthous/Entities/Environment.hs

@@ -0,0 +1,26 @@
+module Xanthous.Entities.Environment
+  ( Wall(..)
+  ) where
+--------------------------------------------------------------------------------
+import Xanthous.Prelude
+import Test.QuickCheck
+import Brick (str)
+import Brick.Widgets.Border.Style (unicode)
+--------------------------------------------------------------------------------
+import Xanthous.Entities (Draw(..), entityIs)
+import Xanthous.Entities.Draw.Util
+import Xanthous.Data
+--------------------------------------------------------------------------------
+
+data Wall = Wall
+  deriving stock (Show, Eq, Ord, Generic, Enum)
+  deriving anyclass (CoArbitrary, Function)
+
+instance Arbitrary Wall where
+  arbitrary = pure Wall
+
+instance Draw Wall where
+  drawWithNeighbors neighs _wall =
+    str . pure . borderFromEdges unicode $ wallEdges
+    where
+      wallEdges = any (entityIs @Wall) <$> edges neighs