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/tests/test_opaque_vectors.rs

@@ -98,10 +98,7 @@ fn populate_test_vectors<CS: CipherSuite>(values: &Value) -> OpaqueTestVectorPar
         dummy_public_key: {
             match decode(values, "client_public_key") {
                 Some(value) => value,
-                None => KeGroup::<CS>::serialize_pk(KeGroup::<CS>::public_key(
-                    KeGroup::<CS>::random_sk(&mut OsRng),
-                ))
-                .to_vec(),
+                None => KeGroup::<CS>::serialize_sk(&KeGroup::<CS>::random_sk(&mut OsRng)).to_vec(),
             }
         },
         dummy_masking_key: {