Refactor lookup_txos() to use zip()

Following Blockstream#89 (comment)

src/new_index/schema.rs

@@ -1030,16 +1030,15 @@ fn get_previous_txos(block_entries: &[BlockEntry]) -> BTreeSet<OutPoint> {
 
 fn lookup_txos(txstore_db: &DB, outpoints: BTreeSet<OutPoint>) -> Result<HashMap<OutPoint, TxOut>> {
  let keys = outpoints.iter().map(TxOutRow::key).collect::<Vec<_>>();
- let mut remain_outpoints = outpoints.into_iter();
  txstore_db
  .multi_get(keys)
  .into_iter()
- .map(|res| {
-  let outpoint = remain_outpoints.next().unwrap();
- match res.unwrap() {
- Some(txo) => Ok((outpoint, deserialize(&txo).expect("failed to parse TxOut"))),
- None => Err(format!("missing txo {}", outpoint).into()),
- }
+ .zip(outpoints)
+ .map(|(res, outpoint)| {
+ let txo = res
+ .unwrap()
+ .ok_or_else(|| format!("missing txo {}", outpoint))?;
+ Ok((outpoint, deserialize(&txo).expect("failed to parse TxOut")))
  })
  .collect()
 }