refactor: Replace fold with try_fold

The and_then combinator performs a kind of bitwise and operation on two
Option types here.  This is useful since the `checked` arithmetic
returns an option thereby accumulating Option types.  Therefore, either
the checked arithmetic operation performs the addition of the unwrapped
accumulator, or it returns None.

Instead of using `and_then` use the provided `try_fold` method which
will short circuit on `None` when the checked arithmetic is used.  Also,
simplify the staring condition using `Amount:ZERO` since this is
logically equivalent to using the first value if one exists.

Lastly, by using the built in `try_fold`, it's possible the performance
will be improved by making use of the short circuit ability instead of
evaluating each item even when the accumulator holds a None type.

units/src/amount/mod.rs

@@ -605,9 +605,7 @@ where
     T: Iterator<Item = Amount>,
 {
     fn checked_sum(mut self) -> Option<Amount> {
-        let first = Some(self.next().unwrap_or_default());
+        self.try_fold(Amount::ZERO, Amount::checked_add)
-
-        self.fold(first, |acc, item| acc.and_then(|acc| acc.checked_add(item)))
     }
 }
 
@@ -616,9 +614,7 @@ where
     T: Iterator<Item = SignedAmount>,
 {
     fn checked_sum(mut self) -> Option<SignedAmount> {
-        let first = Some(self.next().unwrap_or_default());
+        self.try_fold(SignedAmount::ZERO, SignedAmount::checked_add)
-
-        self.fold(first, |acc, item| acc.and_then(|acc| acc.checked_add(item)))
     }
 }