Create a Swift Benchmark from BucketSort

[rajbarik]

Recently I worked on the ExistentialSpecializer optimization in the Swift compiler (swiftlang/swift#19820, others). BucketSort seems to improve significantly with this optimization. BucketSort has nice protocol usages and I was wondering if it makes sense to make it a Swift performance benchmark. What are the licensing and other issues? Any thoughts?

[kelvinlauKL]

We operate under MIT, so you're free to use anything here =)