Improve the performance of sum

Motivation:

`sum(where:)` defined its complexity of O(n) while the implementation was O(2n). An implementation is possible to do the filtering inlined.

In addition, all overloads of `sum` should be inlinable to provide the compiler the most opportunity to optimize these style of calculations.

Modifications:

- Add `@inlinable` attributes to both overloads of `sum`
- Update `sum(where:)` to assert the `isIncluded` predicate and calculating the sum in a single traversal

Result:

Calculating a sum of currencies that pass a filter predicate should be twice as fast.

Sources/Currency/AnyCurrency+Sequence.swift

@@ -27,6 +27,7 @@ extension Sequence where Element: AnyCurrency {
   /// If the sequence has no elements, you will receive a currency with a value of "0".
   /// - Complexity: O(*n*) , where *n* is the length of the sequence.
   /// - Returns: A currency value representing the sum total of all the amounts in the sequence.
+  @inlinable
   public func sum() -> Element {
     return self.reduce(into: .init(.zero), { $0 += $1 })
   }
@@ -35,15 +36,19 @@ extension Sequence where Element: AnyCurrency {
   /// For example:
   ///
   ///     let amounts: [USD] = [304.98, 19.02, 30.21]
-  ///     let sumTotal = amounts.sum(where: { $0.roundedAmount > 20 })
+  ///     let sumTotal = amounts.sum(where: { $0.amount > 20 })
   ///     print(sumTotal)
   ///     // prints "335.19"
   ///
   /// - Complexity: O(*n*), where *n* is the length of the sequence.
-  /// - Parameter predicate: A closure that takes a currency element as its argument
+  /// - Parameter isIncluded: A closure that takes a currency element as its argument
   ///   and returns a Boolean value that indicates whether the passed element should be included in the sum.
-  /// - Returns:A currency value representing the sum total of all the amounts in the sequence that satisfies `predicate`.
-  public func sum(where predicate: (Element) throws -> Bool) rethrows -> Element {
-    return try self.filter(predicate).sum()
+  /// - Returns: A currency value representing the sum total of all the amounts `isIncluded` allowed.
+  @inlinable
+  public func sum(where isIncluded: (Element) throws -> Bool) rethrows -> Element {
+    return try self.reduce(into: Element(0)) { result, next in
+      guard try isIncluded(next) else { return }
+      result += next
+    }
   }
 }