Box the transactions in CombineError

clippy warns of big error variant, box the transactions.

src/v0/error.rs

@@ -309,9 +309,9 @@ pub enum CombineError {
     /// Attempting to combine with a PSBT describing a different unsigned transaction.
     UnexpectedUnsignedTx {
         /// Expected transaction.
-        expected: Transaction,
+        expected: Box<Transaction>,
         /// Actual transaction.
-        actual: Transaction,
+        actual: Box<Transaction>,
     },
     /// Global extended public key has inconsistent key sources.
     InconsistentKeySources(InconsistentKeySourcesError),

src/v0/map/global.rs

@@ -235,8 +235,8 @@ impl Global {
     pub fn combine(&mut self, other: Self) -> Result<(), CombineError> {
         if self.unsigned_tx != other.unsigned_tx {
             return Err(CombineError::UnexpectedUnsignedTx {
-                expected: self.unsigned_tx.clone(),
-                actual: other.unsigned_tx,
+                expected: Box::new(self.unsigned_tx.clone()),
+                actual: Box::new(other.unsigned_tx),
             });
         }