Remove Copy trait implementation from InnerEnvelopeMode (#237)

The Copy trait isn't allowed on types with destructors

src/envelope.rs

@@ -65,7 +65,7 @@ fn recover_keys_internal<CS: CipherSuite>(
     Ok(client_static_keypair)
 }
 
-#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq, Zeroize)]
+#[derive(Clone, Debug, Eq, Hash, PartialEq, Zeroize)]
 #[zeroize(drop)]
 pub(crate) enum InnerEnvelopeMode {
     Zero = 0,
@@ -102,7 +102,7 @@ pub(crate) struct Envelope<CS: CipherSuite> {
 impl<CS: CipherSuite> Clone for Envelope<CS> {
     fn clone(&self) -> Self {
         Self {
-            mode: self.mode,
+            mode: self.mode.clone(),
             nonce: self.nonce.clone(),
             hmac: self.hmac.clone(),
         }