Add Serialize instance for Crop layer

src/Grenade/Layers/Crop.hs

@@ -18,6 +18,7 @@ module Grenade.Layers.Crop (
 
 import           Data.Maybe
 import           Data.Proxy
+import           Data.Serialize
 import           Data.Singletons.TypeLits
 import           GHC.TypeLits
 
@@ -42,6 +43,10 @@ instance UpdateLayer (Crop l t r b) where
   runUpdate _ x _ = x
   createRandom = return Crop
 
+instance Serialize (Crop l t r b) where
+  put _ = return ()
+  get = return Crop
+
 -- | A two dimentional image can be cropped.
 instance ( KnownNat cropLeft
          , KnownNat cropTop