Add cellular-automata cave generator

Add a cellular-automata-based cave level generator, plus an
optparse-applicative-based CLI for invoking level generators in general.

src/Xanthous/Data.hs

@@ -16,6 +16,12 @@ module Xanthous.Data
   , positioned
   , loc
 
+    -- *
+  , Dimensions'(..)
+  , Dimensions
+  , HasWidth(..)
+  , HasHeight(..)
+
     -- *
   , Direction(..)
   , opposite
@@ -88,6 +94,21 @@ loc = iso hither yon
 
 --------------------------------------------------------------------------------
 
+data Dimensions' a = Dimensions
+  { _width :: a
+  , _height :: a
+  }
+  deriving stock (Show, Eq, Functor, Generic)
+  deriving anyclass (CoArbitrary, Function)
+makeFieldsNoPrefix ''Dimensions'
+
+instance Arbitrary a => Arbitrary (Dimensions' a) where
+  arbitrary = Dimensions <$> arbitrary <*> arbitrary
+
+type Dimensions = Dimensions' Word
+
+--------------------------------------------------------------------------------
+
 data Direction where
   Up        :: Direction
   Down      :: Direction