Zip2WithNilPadding

一个针对不同长度序列的Zip2实现，通过nil填充缺失项。

针对Swift 4进行了更新，基于StackOverflow线程中的解决方案：[链接](https://stackoverflow.com/questions/25153477/in-swift-i-would-like-to-join-two-sequences-in-to-a-sequence-of-tuples)

示例

let first: [String] = ["a", "b", "c", "d"]
let second: [Int] = [1, 2]
let actual = Array(Zip2WithNilPadding(first, second))

# [(Optional("a"), Optional(1)), (Optional("b"), Optional(2)), (Optional("c"), Optional(3))]

许可

MIT许可