Implement Sum for an iterator of references to amounts

We have `iter::Sum` already for `Amount` and `SignedAmount`. Add
an implementation for each to support iterators that yield references.

units/src/amount/signed.rs

@@ -442,6 +442,16 @@ impl core::iter::Sum for SignedAmount {
     }
 }
 
+impl<'a> core::iter::Sum<&'a SignedAmount> for SignedAmount {
+    fn sum<I>(iter: I) -> Self
+    where
+        I: Iterator<Item = &'a SignedAmount>,
+    {
+        let sats: i64 = iter.map(|amt| amt.0).sum();
+        SignedAmount::from_sat(sats)
+    }
+}
+
 #[cfg(feature = "arbitrary")]
 impl<'a> Arbitrary<'a> for SignedAmount {
     fn arbitrary(u: &mut Unstructured<'a>) -> arbitrary::Result<Self> {

units/src/amount/tests.rs

@@ -825,8 +825,8 @@ fn serde_as_sat_opt() {
 
 #[test]
 fn sum_amounts() {
-    assert_eq!(Amount::from_sat(0), [].into_iter().sum::<Amount>());
-    assert_eq!(SignedAmount::from_sat(0), [].into_iter().sum::<SignedAmount>());
+    assert_eq!(Amount::from_sat(0), [].iter().sum::<Amount>());
+    assert_eq!(SignedAmount::from_sat(0), [].iter().sum::<SignedAmount>());
 
     let amounts = [Amount::from_sat(42), Amount::from_sat(1337), Amount::from_sat(21)];
     let sum = amounts.into_iter().sum::<Amount>();

units/src/amount/unsigned.rs

@@ -432,6 +432,16 @@ impl core::iter::Sum for Amount {
     }
 }
 
+impl<'a> core::iter::Sum<&'a Amount> for Amount {
+    fn sum<I>(iter: I) -> Self
+    where
+        I: Iterator<Item = &'a Amount>,
+    {
+        let sats: u64 = iter.map(|amt| amt.0).sum();
+        Amount::from_sat(sats)
+    }
+}
+
 #[cfg(feature = "arbitrary")]
 impl<'a> Arbitrary<'a> for Amount {
     fn arbitrary(u: &mut Unstructured<'a>) -> arbitrary::Result<Self> {