Add stableSorted to Swift collections

RandomAccessCollection+StableSorted.swift

extension RandomAccessCollection {
    func stableSorted(by areInIncreasingOrder: (Self.Element, Self.Element) -> Bool) -> [Self.Element] {
        return enumerated()
            .sorted { first, second in
                return areInIncreasingOrder(first.element, second.element) || first.offset < second.offset
            }
            .map { $0.element }
    }
}