move Nat specific to Cryptonite's insternal module

Crypto/Hash/SHAKE.hs

@@ -73,10 +73,7 @@ shakeFinalizeOutput :: (IsDivisibleBy8 bitLen, KnownNat bitLen)
                     -> IO ()
 shakeFinalizeOutput d ctx dig = do
     c_sha3_finalize_shake ctx
-    c_sha3_output ctx dig (fromInteger (natVal d `div` 8))
-
-byteLen :: (KnownNat bitlen, IsDivisibleBy8 bitlen, Num a) => proxy bitlen -> a
-byteLen d = fromInteger (natVal d `div` 8)
+    c_sha3_output ctx dig (byteLen d)
 
 foreign import ccall unsafe "cryptonite_sha3_init"
     c_sha3_init :: Ptr (Context a) -> Word32 -> IO ()

Crypto/Internal/Nat.hs

@@ -6,10 +6,18 @@
 {-# LANGUAGE UndecidableInstances #-}
 module Crypto.Internal.Nat
     ( type IsDivisibleBy8
+    , byteLen
+    , integralNatVal
     ) where
 
 import           GHC.TypeLits
 
+byteLen :: (KnownNat bitlen, IsDivisibleBy8 bitlen, Num a) => proxy bitlen -> a
+byteLen d = fromInteger (natVal d `div` 8)
+
+integralNatVal :: (KnownNat bitlen, Num a) => proxy bitlen -> a
+integralNatVal = fromInteger . natVal
+
 type family IsDiv8 (bitLen :: Nat) (n :: Nat) where
     IsDiv8 bitLen 0 = 'True
 #if MIN_VERSION_base(4,9,0)