Add Sequence.reduce(_:) to complete the reductions family

[ZayanKhan-12]

Description

Fixes #241. Of the three reductions overloads on Sequence, two mirror a standard-library reduce overload (reduce(_:_:) ↔ reductions(_:_:), reduce(into:_:) ↔ reductions(into:_:)), but reductions(_:) — which seeds the operation with the first element — had no single-value counterpart. This adds it:

let total = (1...5).reduce(+)
// total == 15

let none = EmptyCollection<Int>().reduce(+)
// none == nil

The overload returns nil for an empty sequence, per the issue. It is implemented with rethrows for consistency with the rest of the package (the typed-throws variant sketched in #241 requires a Swift 6 minimum and an untyped→typed error cast; happy to switch if that's preferred).

Detailed Design

extension Sequence {
  @inlinable
  public func reduce(
    _ nextPartialResult: (Element, Element) throws -> Element
  ) rethrows -> Element?
}

No lazy variant is needed — the result is a single value, not a sequence.

Testing

ReduceTests covers: multi-element/single-element/empty sequences, agreement with reductions(_:).last, left-to-right association for non-commutative operations, single-pass sequence consumption, and error propagation (including that the closure is never invoked for 0- or 1-element sequences). Full suite: 228 tests pass.

Documentation

Added reduce(_:) to the Reductions documentation page with an overview example.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

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