Add bytestring dependency

byg/byg.cabal

@@ -20,3 +20,4 @@ executable byg
     build-depends:
         base
       , mtl
+      , bytestring

byg/src/Main.hs

@@ -6,6 +6,7 @@ module Main where
 import Unsafe.Coerce (unsafeCoerce)
 import Control.Monad.State
 import Control.Monad.Writer
+import Data.ByteString (ByteString)
 
 data Source a where
   -- Void :: Source ()