Implement Script for Witness and Add Tweak in Partially Signed Bitcoi…

…n Transaction to make it a single data source needed for a Signer to produce valid signatures.

lightning/src/sign/mod.rs

@@ -445,7 +445,7 @@ impl SpendableOutputDescriptor {
 					key: "add_tweak".as_bytes().to_vec(),
 				},
 				add_tweak,
-			)].into_iter().collect()}).or(Some(Default::default())).expect("proprietary fields"),
+			)].into_iter().collect()}).unwrap_or_default(),
 			unknown: Default::default(),
 		};
 		Ok((psbt, expected_max_weight))