Merge #996: Box value encoded in a variant to reduce enum stack space

9906cea Box value encoded in a variant to reduce enum stack space (Riccardo Casatta)

Pull request description:

  before

  ```
  print-type-size type: `util::psbt::error::Error`: 120 bytes, alignment: 8 bytes
  print-type-size     discriminant: 1 bytes
  print-type-size     variant `CombineInconsistentKeySources`: 115 bytes
  print-type-size         padding: 3 bytes
  print-type-size         field `.0`: 112 bytes, alignment: 4 bytes
  print-type-size     variant `InvalidKey`: 39 bytes
  print-type-size         padding: 7 bytes
  print-type-size         field `.0`: 32 bytes, alignment: 8 bytes
  ```

  after
  ```
  print-type-size type: `util::psbt::error::Error`: 40 bytes, alignment: 8 bytes
  print-type-size     discriminant: 1 bytes
  print-type-size     variant `InvalidKey`: 39 bytes
  print-type-size         padding: 7 bytes
  print-type-size         field `.0`: 32 bytes, alignment: 8 bytes
  print-type-size     variant `DuplicateKey`: 39 bytes
  print-type-size         padding: 7 bytes
  print-type-size         field `.0`: 32 bytes, alignment: 8 bytes
  ```

  `util::psbt::error::Error` is wrapped also in `consensus::encode::Error` and stack savings are gained there also

ACKs for top commit:
  apoelstra:
    ACK 9906cea
  tcharding:
    ACK 9906cea
  sanket1729:
    utACK 9906cea

Tree-SHA512: e03988fcbc3dd87f83d00dd84ec1c538bc5c63bea97ff4a69a715621f498f57d7fe2a623e351942d9532af40c723e42a9eb6ef48ebf4c62ddf5c0f44e9ea0a07

src/util/psbt/error.rs

@@ -80,7 +80,7 @@ pub enum Error {
     },
     /// Conflicting data during combine procedure:
     /// global extended public key has inconsistent key sources
-    CombineInconsistentKeySources(ExtendedPubKey),
+    CombineInconsistentKeySources(Box<ExtendedPubKey>),
     /// Serialization error in bitcoin consensus-encoded structures
     ConsensusEncoding,
 }

src/util/psbt/mod.rs

@@ -192,7 +192,7 @@ impl PartiallySignedTransaction {
                         entry.insert((fingerprint1, derivation1));
                         continue
                     }
-                    return Err(Error::CombineInconsistentKeySources(xpub));
+                    return Err(Error::CombineInconsistentKeySources(Box::new(xpub)));
                 }
             }
         }