Add a bindings-only version of Future::register_callback

While we could, in theory, add support to the bindings logic to map
`Box<dyn Trait>`, there isn't a whole lot of use doing so when its
incredibly trivial to do directly.

This adds a trivial wrapper around `Future::register_callback` that
is only built in bindings and which is linked in the
`register_callback` docs for visibility.

Author: TheBlueMatt

lightning/src/util/wakers.rs

@@ -163,6 +163,8 @@ pub struct Future {
 impl Future {
 	/// Registers a callback to be called upon completion of this future. If the future has already
 	/// completed, the callback will be called immediately.
+	///
+	/// (C-not exported) use the bindings-only `register_callback_fn` instead
 	pub fn register_callback(&self, callback: Box<dyn FutureCallback>) {
 		let mut state = self.state.lock().unwrap();
 		if state.complete {
@@ -172,6 +174,16 @@ impl Future {
 			state.callbacks.push(callback);
 		}
 	}
+
+	// C bindings don't (currently) know how to map `Box<dyn Trait>`, and while it could add the
+	// following wrapper, doing it in the bindings is currently much more work than simply doing it
+	// here.
+	/// Registers a callback to be called upon completion of this future. If the future has already
+	/// completed, the callback will be called immediately.
+	#[cfg(c_bindings)]
+	pub fn register_callback_fn<F: 'static + FutureCallback>(&self, callback: F) {
+		self.register_callback(Box::new(callback));
+	}
 }
 
 mod std_future {