Expose ability to extract supervisor from actor cell

Resolves #294

ractor/src/actor/actor_cell.rs

@@ -541,6 +541,14 @@ impl ActorCell {
         self.inner.tree.stop_all_children(reason);
     }
 
+    /// Tries to retrieve this actor's supervisor. 
+    /// 
+    /// Returns [None] if this actor has no supervisor at the given instance or 
+    /// [Some(ActorCell)] supervisor if one is configured.
+    pub fn try_get_superivisor(&self) -> Option<ActorCell> {
+        self.inner.tree.try_get_supervisor()
+    }
+
     /// Stop any children of this actor, and wait for their collective exit, optionally
     /// threading the optional reason to all children
     ///

ractor/src/actor/supervision.rs

@@ -48,6 +48,11 @@ impl SupervisionTree {
         *(self.supervisor.lock().unwrap()) = None;
     }
 
+    /// Try and retrieve the set supervisor
+    pub(crate) fn try_get_supervisor(&self) -> Option<ActorCell> {
+        self.supervisor.lock().unwrap().clone()
+    }
+
     /// Terminate all your supervised children and unlink them
     /// from the supervision tree since the supervisor is shutting down
     /// and can't deal with superivison events anyways

ractor/src/actor/tests/supervisor.rs

@@ -97,6 +97,10 @@ async fn test_supervision_panic_in_post_startup() {
         .await
         .expect("Child panicked on startup");
 
+    let maybe_sup = child_ref.try_get_superivisor();
+    assert!(maybe_sup.is_some());
+    assert_eq!(maybe_sup.map(|a| a.get_id()), Some(supervisor_ref.get_id()));
+
     let (_, _) = tokio::join!(s_handle, c_handle);
 
     assert_eq!(child_ref.get_id().pid(), flag.load(Ordering::Relaxed));