support prompt in message serialization

That seems to be the last thing needed for message serialization.
Although it's only used in the assistant currently, so hard to tell if I
forgot something.

At this point, it should be possible to start using transferkeys
when performing transfers, which will allow killing a transferkeys
process if a transfer times out or stalls. But that's for another day.

This commit was sponsored by Ethan Aubin.

Messages/Progress.hs

@@ -113,7 +113,7 @@ metered' st othermeter msize showoutput a = go st
 				updateMeter meter
 			a meter (combinemeter m)
 		| otherwise = nometer
-	go (MessageState { outputType = SerializedOutput h }) = do
+	go (MessageState { outputType = SerializedOutput h _ }) = do
 		liftIO $ outputSerialized h $ StartProgressMeter msize
 		meter <- liftIO $ mkMeter msize $ \_ _ _old new ->
 			outputSerialized h $ UpdateProgressMeter $