Ensure seeds cannot be printed

We don't want to display (or debug out) seeds.   Implement dummy methods for
Display trait and Debug.  Add test to ensure the dummy implementations don't get
changed.

Fixes: #722

Signed-off-by: tcharding <me@tobin.cc>

application/comit_node/src/seed.rs

@@ -10,7 +10,13 @@ pub struct Seed(#[serde(with = "hex_serde")] [u8; SEED_LENGTH]);
 
 impl fmt::Debug for Seed {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "Seed")
+        write!(f, "do not implement")
+    }
+}
+
+impl fmt::Display for Seed {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "do not implement")
     }
 }
 
@@ -68,4 +74,14 @@ mod tests {
 
         assert_ne!(random1, random2);
     }
+
+    #[test]
+    fn test_display_and_debug_not_implemented() {
+        let seed = Seed::new_random().unwrap();
+
+        let out = format!("{}", seed);
+        assert_eq!(out, "do not implement".to_string());
+        let debug = format!("{:?}", seed);
+        assert_eq!(debug, "do not implement".to_string());
+    }
 }