Generalize callbacks for subscriptions (#260)

* Generalize callbacks to subscriptions

By implementing a trait (SubscriptionCallback) on valid function signatures,
and making subscriptions accept callbacks that implement this trait, we can now

* Receive both plain and boxed messages
* Optionally receive a MessageInfo along with the message, as the second argument
* Soon, receive a loaned message instead of an owned one

This corresponds to the functionality in any_subscription_callback.hpp in rclcpp.

Author: nnmm

rosidl_runtime_rs/src/traits.rs

@@ -144,20 +144,6 @@ pub trait Message: Clone + Debug + Default + 'static + Send + Sync {
     fn from_rmw_message(msg: Self::RmwMsg) -> Self;
 }
 
-/// Allows boxed values to be used by users in Publishers and Subscriptions.
-// See https://github.com/ros2-rust/ros2_rust/pull/235 for a discussion of possible alternatives.
-impl<T: Message> Message for Box<T> {
-    type RmwMsg = T::RmwMsg;
-
-    fn into_rmw_message(msg_cow: Cow<'_, Self>) -> Cow<'_, Self::RmwMsg> {
-        T::into_rmw_message(Cow::Owned(*msg_cow.into_owned()))
-    }
-
-    fn from_rmw_message(msg: Self::RmwMsg) -> Self {
-        Box::new(T::from_rmw_message(msg))
-    }
-}
-
 /// Trait for services.
 ///
 /// User code never needs to call this trait's method, much less implement this trait.