gstreamer: manually implement Debug on PromiseReply

The default implementation was not very useful:

PromiseReply(
    Promise {
        inner: Shared {
            inner: 0x00007ff108001090,
        },
    },
)

gstreamer/src/promise.rs

@@ -150,7 +150,6 @@ unsafe impl Sync for Promise {}
 #[derive(Debug)]
 pub struct PromiseFuture(Promise, futures_channel::oneshot::Receiver<()>);
 
-#[derive(Debug)]
 pub struct PromiseReply(Promise);
 
 impl std::future::Future for PromiseFuture {
@@ -196,6 +195,18 @@ impl Deref for PromiseReply {
     }
 }
 
+impl std::fmt::Debug for PromiseReply {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let mut debug = f.debug_tuple("PromiseReply");
+
+        match self.0.get_reply() {
+            Some(reply) => debug.field(reply),
+            None => debug.field(&"<no reply>"),
+        }
+        .finish()
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;