Avoid ref to a Vec when accessing custom onion TLVs

The bindings can't easily return a reference to a `Vec`, so we
instead simply return a slice.

Author: TheBlueMatt

lightning/src/ln/outbound_payment.rs

@@ -573,7 +573,7 @@ impl RecipientOnionFields {
 	/// Note that if this field is non-empty, it will contain strictly increasing TLVs, each
 	/// represented by a `(u64, Vec<u8>)` for its type number and serialized value respectively.
 	/// This is validated when setting this field using [`Self::with_custom_tlvs`].
-	pub fn custom_tlvs(&self) -> &Vec<(u64, Vec<u8>)> {
+	pub fn custom_tlvs(&self) -> &[(u64, Vec<u8>)] {
 		&self.custom_tlvs
 	}