Place doors on the level

Pick a random subset of cells on the level that have a wall on two
opposite sides and are clear on the other two sides, and place closed,
unlocked doors on those cells.

src/Xanthous/Entities/Environment.hs

@@ -7,6 +7,7 @@ module Xanthous.Entities.Environment
   , Door(..)
   , open
   , locked
+  , unlockedDoor
     -- * Messages
   , GroundMessage(..)
   ) where
@@ -88,6 +89,13 @@ instance Entity Door where
   description _ = "a door"
   entityChar _ = "d"
 
+-- | A closed, unlocked door
+unlockedDoor :: Door
+unlockedDoor = Door
+  { _open = False
+  , _locked = False
+  }
+
 --------------------------------------------------------------------------------
 
 newtype GroundMessage = GroundMessage Text