Add Show instances for EdDSA secret keys

Other algorithms define Show instances for their secrets.
Here ScrubbedBytes will obfuscate the content anyway.

Will be useful for X509.PrivKey, which requires a Show instance.

Crypto/PubKey/Ed25519.hs

@@ -42,7 +42,7 @@ import           Crypto.Random
 
 -- | An Ed25519 Secret key
 newtype SecretKey = SecretKey ScrubbedBytes
-    deriving (Eq,ByteArrayAccess,NFData)
+    deriving (Show,Eq,ByteArrayAccess,NFData)
 
 -- | An Ed25519 public key
 newtype PublicKey = PublicKey Bytes

Crypto/PubKey/Ed448.hs

@@ -46,7 +46,7 @@ import           Crypto.Random
 
 -- | An Ed448 Secret key
 newtype SecretKey = SecretKey ScrubbedBytes
-    deriving (Eq,ByteArrayAccess,NFData)
+    deriving (Show,Eq,ByteArrayAccess,NFData)
 
 -- | An Ed448 public key
 newtype PublicKey = PublicKey Bytes