Keypair: implement clone()

This was not implemented upstream in ed25519-dalek to force everyone to
think twice before creating another copy of a potentially sensitive
private key in memory.

See dalek-cryptography/ed25519-dalek#76

However, there are now 9 instances of
  Keypair::from_bytes(&keypair.to_bytes())
in the solana codebase and it would be preferable to have a function.

In particular since this also comes up when writing programs and can
cause users to either start messing with lifetimes or discover the
from_bytes() workaround themselves.

This patch opts to not implement the Clone trait. This avoids automatic
use in order to preserve some of the original "let developers think
twice about this" intention.

sdk/src/signer/keypair.rs

@@ -97,6 +97,26 @@ where
     }
 }
 
+/// Allows Keypair cloning
+///
+/// Note that the `Clone` trait is intentionally unimplemented because making a
+/// second copy of sensitive secret keys in memory is usually a bad idea.
+///
+/// Only use this in tests or when strictly required.
+pub trait KeypairInsecureClone {
+    fn clone(&self) -> Self;
+}
+
+impl KeypairInsecureClone for Keypair {
+    fn clone(&self) -> Self {
+        Self(ed25519_dalek::Keypair {
+            // This will never error since self is a valid keypair
+            secret: ed25519_dalek::SecretKey::from_bytes(self.0.secret.as_bytes()).unwrap(),
+            public: self.0.public,
+        })
+    }
+}
+
 /// Reads a JSON-encoded `Keypair` from a `Reader` implementor
 pub fn read_keypair<R: Read>(reader: &mut R) -> Result<Keypair, Box<dyn error::Error>> {
     let bytes: Vec<u8> = serde_json::from_reader(reader)?;