Add .child() function to allow topic to act as prefix

Author: The-Ladle

src/topic.rs

@@ -109,6 +109,11 @@ impl Topic {
         &mut self.name
     }
 
+    /// Creates a child topic
+    pub fn child(&self, name: String){
+        Topic::new(self.name.clone() + &*name, self.handle.clone());
+    }
+
     /// Publishes to this topic with the data type `T`.
     ///
     /// For a generic-free version, see [`generic_publish`][`Self::generic_publish`].