[signalapp#472] remove Debug from SessionRecord

- also add a cheap Debug impl for PrivateKey which leaks no info

rust/protocol/src/curve.rs

@@ -297,6 +297,14 @@ impl TryFrom<&[u8]> for PrivateKey {
     }
 }
 
+impl fmt::Debug for PrivateKey {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        /* Calculating the equivalent public key isn't free, and we want to discourage printing out
+         * the actual private key data, so we just provide the key type here. */
+        write!(f, "PrivateKey {{ key_type={} }}", self.key_type())
+    }
+}
+
 #[derive(Copy, Clone)]
 pub struct KeyPair {
     pub public_key: PublicKey,

rust/protocol/src/state/session.rs

@@ -466,7 +466,7 @@ impl From<&SessionState> for SessionStructure {
     }
 }
 
-#[derive(Clone, Debug)]
+#[derive(Clone)]
 pub struct SessionRecord {
     current_session: Option<SessionState>,
     previous_sessions: Vec<Vec<u8>>,