Implement `CheckedSum` for amount types

It's just `Sum` with checked arithmetic.

src/util/amount.rs

@@ -863,6 +863,35 @@ impl ::std::iter::Sum for SignedAmount {
     }
 }
 
+/// Calculate the sum over the iterator using checked arithmetic.
+pub trait CheckedSum<R> {
+    /// Calculate the sum over the iterator using checked arithmetic. If an over or underflow would
+    /// happen it returns `None`.
+    fn checked_sum(self) -> Option<R>;
+}
+
+impl<T> CheckedSum<Amount> for T where T: Iterator<Item = Amount> {
+    fn checked_sum(mut self) -> Option<Amount> {
+        let first = Some(self.next().unwrap_or_default());
+
+        self.fold(
+            first,
+            |acc, item| acc.and_then(|acc| acc.checked_add(item))
+        )
+    }
+}
+
+impl<T> CheckedSum<SignedAmount> for T where T: Iterator<Item = SignedAmount> {
+    fn checked_sum(mut self) -> Option<SignedAmount> {
+        let first = Some(self.next().unwrap_or_default());
+
+        self.fold(
+            first,
+            |acc, item| acc.and_then(|acc| acc.checked_add(item))
+        )
+    }
+}
+
 #[cfg(feature = "serde")]
 pub mod serde {
     // methods are implementation of a standardized serde-specific signature
@@ -1548,4 +1577,50 @@ mod tests {
         let sum = amounts.into_iter().sum::<SignedAmount>();
         assert_eq!(SignedAmount::from_sat(1316), sum);
     }
+
+    #[test]
+    fn checked_sum_amounts() {
+        assert_eq!(Some(Amount::from_sat(0)), vec![].into_iter().checked_sum());
+        assert_eq!(Some(SignedAmount::from_sat(0)), vec![].into_iter().checked_sum());
+
+        let amounts = vec![
+            Amount::from_sat(42),
+            Amount::from_sat(1337),
+            Amount::from_sat(21)
+        ];
+        let sum = amounts.into_iter().checked_sum();
+        assert_eq!(Some(Amount::from_sat(1400)), sum);
+
+        let amounts = vec![
+            Amount::from_sat(u64::max_value()),
+            Amount::from_sat(1337),
+            Amount::from_sat(21)
+        ];
+        let sum = amounts.into_iter().checked_sum();
+        assert_eq!(None, sum);
+
+        let amounts = vec![
+            SignedAmount::from_sat(i64::min_value()),
+            SignedAmount::from_sat(-1),
+            SignedAmount::from_sat(21)
+        ];
+        let sum = amounts.into_iter().checked_sum();
+        assert_eq!(None, sum);
+
+        let amounts = vec![
+            SignedAmount::from_sat(i64::max_value()),
+            SignedAmount::from_sat(1),
+            SignedAmount::from_sat(21)
+        ];
+        let sum = amounts.into_iter().checked_sum();
+        assert_eq!(None, sum);
+
+        let amounts = vec![
+            SignedAmount::from_sat(42),
+            SignedAmount::from_sat(3301),
+            SignedAmount::from_sat(21)
+        ];
+        let sum = amounts.into_iter().checked_sum();
+        assert_eq!(Some(SignedAmount::from_sat(3364)), sum);
+    }
 }