Merge #630: fix: needless borrow (reference is immediately dereferenced by the rust-compiler)

d8abb08 fix: needless borrow (wthrajat)

Pull request description:

  fixes:
  ```shell
  this expression creates a reference which is immediately dereferenced by the compiler
     --> src/psbt/mod.rs:881:54
      |
  881 |                     cache.p2wpkh_signature_hash(idx, &script_code, amt, hash_ty)?
      |                                                      ^^^^^^^^^^^^
  ```

ACKs for top commit:
  apoelstra:
    ACK d8abb08

Tree-SHA512: d8975f076e2febad6093513db65af4641d4f29d6e5f499a70f3c7345699f3102041380ede170d1ea49b64e617d2f84950b8a2ae3604739dcf18d20d536e62eb0

Author: apoelstra

src/psbt/mod.rs

@@ -878,7 +878,7 @@ impl PsbtExt for Psbt {
                         .redeem_script
                         .as_ref()
                         .expect("redeem script non-empty checked earlier");
-                    cache.p2wpkh_signature_hash(idx, &script_code, amt, hash_ty)?
+                    cache.p2wpkh_signature_hash(idx, script_code, amt, hash_ty)?
                 } else {
                     let witness_script = inp
                         .witness_script