Clear clippy manual_map warning

The lint `manual_map` is new so we cannot explicitly allow it and
maintain backwards comparability. Instead, allow all lints for
`get_utxo_for` with a comment explaining why.

Author: tcharding

src/psbt/mod.rs

@@ -17,6 +17,7 @@ pub trait PsbtUtils {
 }
 
 impl PsbtUtils for Psbt {
+    #[allow(clippy::all)] // We want to allow `manual_map` but it is too new.
     fn get_utxo_for(&self, input_index: usize) -> Option<TxOut> {
         let tx = &self.global.unsigned_tx;