Strict byteArray of IV c

This provides a way to seq `IV c` and its content when client wants to
prevent thunk exploding, It's not rare to apply multiple `ivAdd` on
single `IV c` and we often don't want thunk here.

Crypto/Cipher/Types/Block.hs

@@ -50,7 +50,7 @@ import           Foreign.Ptr
 import           Foreign.Storable
 
 -- | an IV parametrized by the cipher
-data IV c = forall byteArray . ByteArray byteArray => IV byteArray
+data IV c = forall byteArray . ByteArray byteArray => IV !byteArray
 
 instance BlockCipher c => ByteArrayAccess (IV c) where
     withByteArray (IV z) f = withByteArray z f