No-export &self methods on non-cloneable enum(s)

Specifically, `OnionMessageContents` is a non-cloneable enum, which
isn't stored opaque so we cannot call `&self` methods on it.
Because its methods aren't critical to the API for now, we simply
no-export them rather than trying to work out an alternative
approach.

Author: TheBlueMatt

lightning/src/onion_message/packet.rs

@@ -118,13 +118,16 @@ pub enum OnionMessageContents<T: CustomOnionMessageContents> {
 
 impl<T: CustomOnionMessageContents> OnionMessageContents<T> {
 	/// Returns the type that was used to decode the message payload.
+	///
+	/// (C-not exported) as methods on non-cloneable enums are not currently exportable
 	pub fn tlv_type(&self) -> u64 {
 		match self {
 			&OnionMessageContents::Custom(ref msg) => msg.tlv_type(),
 		}
 	}
 }
 
+/// (C-not exported) as methods on non-cloneable enums are not currently exportable
 impl<T: CustomOnionMessageContents> Writeable for OnionMessageContents<T> {
 	fn write<W: Writer>(&self, w: &mut W) -> Result<(), io::Error> {
 		match self {