feat: derive Clone for KeyPair and XKey

Allow KeyPair and XKey structs to be cloned. We have no technical reason
not to do this, since it is trivial to copy out a private key from
either key type and the underlying members of the struct are all already
Clone.

src/lib.rs

@@ -88,6 +88,7 @@ type Result<T> = std::result::Result<T, crate::error::Error>;
 
 /// The main interface used for reading and writing _nkey-encoded_ key pairs, including
 /// seeds and public keys. Instances of this type cannot be cloned.
+#[derive(Clone)]
 pub struct KeyPair {
     kp_type: KeyPairType,
     sk: Option<SecretKey>, //rawkey_kind: RawKeyKind,

src/xkeys.rs

@@ -18,6 +18,7 @@ use rand::{CryptoRng, Rng, RngCore};
 
 /// The main interface used for reading and writing _nkey-encoded_ curve key
 /// pairs. Instances of this type cannot be cloned.
+#[derive(Clone)]
 pub struct XKey {
     public: PublicKey,
     secret: Option<SecretKey>,