Add SenderKeyName::sender_name, sender_device_id

Avoids an unnecessary clone when getting the device ID since otherwise
we call sender() which clones the ProtocolAddress but then we extract
the u32 and throw away the string.

Rename SenderKeyRecord::empty to is_empty to match Java.

src/sender_keys.rs

@@ -24,6 +24,14 @@ impl SenderKeyName {
         Ok(self.group_id.clone())
     }
 
+    pub fn sender_name(&self) -> Result<String> {
+        Ok(self.sender.name().to_string())
+    }
+
+    pub fn sender_device_id(&self) -> Result<u32> {
+        Ok(self.sender.device_id())
+    }
+
     pub fn sender(&self) -> Result<ProtocolAddress> {
         Ok(self.sender.clone())
     }
@@ -280,7 +288,7 @@ impl SenderKeyRecord {
         Ok(Self { states })
     }
 
-    pub fn empty(&self) -> Result<bool> {
+    pub fn is_empty(&self) -> Result<bool> {
         Ok(self.states.is_empty())
     }