if in doubt, prefer unwrap_or_default()

if the past we had lots of crashes because of unexpected unwrap failures,
mostly related to string.
this commit avoids them eg. for string-conversions that may panic
eg. when encountering a null-byte or by logical programming errors
where an object is assumed to be set but is not under unexpected circumstances.

src/key.rs

@@ -163,8 +163,8 @@ impl Key {
 
     pub fn to_bytes(&self) -> Vec<u8> {
         match self {
-            Key::Public(k) => k.to_bytes().unwrap(),
-            Key::Secret(k) => k.to_bytes().unwrap(),
+            Key::Public(k) => k.to_bytes().unwrap_or_default(),
+            Key::Secret(k) => k.to_bytes().unwrap_or_default(),
         }
     }