message: add generate to the SecretKey
diff: @@ -98,6 +98,9 @@ impl<A: AsymmetricKey> From<SecretKey<A>> for KeyPair<A> {
  }
  impl<A: AsymmetricKey> SecretKey<A> {
  + pub fn generate<T: RngCore + CryptoRng>(rng: T) -> Self {
  + SecretKey(A::generate(rng))
  + }
  pub fn to_public(&self) -> PublicKey<A> {
  PublicKey(<A as AsymmetricKey>::compute_public(&self.0))
  }
  