Remove Zeroize Tests (#381)

* Remove Zeroize tests

* Remove `Zeroize` requirements from `Group::Pk`

* Remove `Copy` requirements from `Group::Pk`

* Change to `ZeroizeOnDrop` requirements for `Group::Sk`

* Add note why we don't use `elliptic_curve::PublicKey`

src/key_exchange/sigma_i/message.rs

@@ -192,12 +192,18 @@ impl<CS: CipherSuite, KE: Group> CachedMessage<CS, KE> {
     derive(serde::Deserialize, serde::Serialize),
     serde(bound = "")
 )]
-#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq, Zeroize)]
+#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
 pub(super) enum Role {
     Server,
     Client,
 }
 
+impl Zeroize for Role {
+    fn zeroize(&mut self) {
+        *self = Self::Server;
+    }
+}
+
 enum Stage {
     Sign,
     Verify,