Add mapping sum overload

Motivation:

It is common to want a sum of values after applying a transform, but doing so with a map and sum is O(2n) while an implementation could be made for O(n).

Modifications:

- Add `sum(_:)` overload for inline transformations before summing.

Result:

Faster sums of transformations

Sources/Currency/AnyCurrency+Sequence.swift

@@ -14,6 +14,8 @@
 
 import Foundation
 
+// MARK: Sum
+
 extension Sequence where Element: AnyCurrency {
   /// Returns the sum total of all amounts in the sequence.
   ///
@@ -51,4 +53,24 @@ extension Sequence where Element: AnyCurrency {
       result += next
     }
   }
+
+  /// Returns the sum total of amounts in the sequence after applying the provided transform.
+  ///
+  /// Rather than doing a `.map(_:)` and then `.sum()`, the `sum` result will be calculated inline while applying the transformations.
+  ///
+  /// For example, you may want to calculate what the total taxes would be from a sequence of individual prices:
+  ///
+  ///     let prices: [USD] = [3.00, 2.99, 5.98]
+  ///     // apply a 9% tax rate to each price and calculate the sum
+  ///     let totalTaxes = prices.sum { $0 * Decimal(0.09) }
+  ///     // totalTaxes == USD(1.08)
+  ///
+  /// - Complexity: O(*n*), where *n* is the length of the sequence.
+  /// - Parameter transform: A mapping closure. `transform` accepts an element of this sequence as its parameter
+  /// and returns a transformed value of the same type.
+  /// - Returns: A currency value representing the sum total of all the transformed amounts in the sequence.
+  @inlinable
+  public func sum(_ transform: (Element) throws -> (Element)) rethrows -> Element {
+    return try self.reduce(into: .init(.zero)) { $0 += try transform($1) }
+  }
 }

Tests/CurrencyTests/AnyCurrencyTests.swift

@@ -182,11 +182,6 @@ extension Sequence where Element: AnyCurrency {
   func applyingRate(_ rate: Decimal) -> [Element] {
     return self.reduce(into: [Element]()) { $0.append($1 + ($1 * rate)) }
   }
-
-  func sum(_ transform: (Element) -> Element) -> Element {
-    return self.reduce(into: [Element](), { $0.append(transform($1)) })
-      .sum()
-  }
 }
 
 extension AnyCurrencyTests {