♻️ zb: replace some Node::at() usage with Node::add_interface()

Node::at() isn't a very friendly API. The closure isn't necessary in
normal situation (only in error case will it actually save a few cycles,
eventually).

Provide a simpler Node::add_interface() method and use it for standard
interfaces registration.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

zbus/src/object_server/mod.rs

@@ -194,9 +194,9 @@ impl Node {
             path,
             ..Default::default()
         };
-        node.at(Peer::name(), || ArcInterface::new(Peer));
-        node.at(Introspectable::name(), || ArcInterface::new(Introspectable));
-        node.at(Properties::name(), || ArcInterface::new(Properties));
+        debug_assert!(node.add_interface(Peer));
+        debug_assert!(node.add_interface(Introspectable));
+        debug_assert!(node.add_interface(Properties));
 
         node
     }
@@ -277,6 +277,23 @@ impl Node {
         self.children.remove(node).is_some()
     }
 
+    fn add_arc_interface(&mut self, name: InterfaceName<'static>, arc_iface: ArcInterface) -> bool {
+        match self.interfaces.entry(name) {
+            Entry::Vacant(e) => {
+                e.insert(arc_iface);
+                true
+            }
+            Entry::Occupied(_) => false,
+        }
+    }
+
+    fn add_interface<I>(&mut self, iface: I) -> bool
+    where
+        I: Interface,
+    {
+        self.add_arc_interface(I::name(), ArcInterface::new(iface))
+    }
+
     // Takes a closure so caller can avoid having to create an Arc & RwLock in case interface was
     // already added.
     fn at<F>(&mut self, name: InterfaceName<'static>, iface_creator: F) -> bool