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/Character.hs

@@ -2,14 +2,14 @@ module Xanthous.Entities.Character
   ( Character(..)
   , mkCharacter
   ) where
-
+--------------------------------------------------------------------------------
 import Xanthous.Prelude
 import Test.QuickCheck
-
+--------------------------------------------------------------------------------
 import Xanthous.Entities
+--------------------------------------------------------------------------------
 
-data Character where
-  Character :: Character
+data Character = Character
   deriving stock (Show, Eq, Ord, Generic)
   deriving anyclass (CoArbitrary, Function)
   deriving Draw via (DrawCharacter "@" Character)