[Rotate] Non-mutating variants for rotating APIs

[LucianoPAlmeida]

This is an implementation of non-mutating variant for rotate(toStartAt:) and rotate(subrange:toStartAt:) that are rotated(toStartAt:) and rotated(subrange:toStartAt:) respectively that results in a RotatedCollection view. The useful aspects of such API is that when preserving the original collection, the mutating version requires a copy. Also, when base conforms to RandomAccessCollection calling rotated is O(1) time and elements at each position are also computed in constant time.

Couple of other points to note:

• Rotate.md documentation was updated with new variants. Not sure if was worth a new file both for implementation or docs. So opted to keep everything together.
• Tests to ensure that both mutating and non-mutating variants produce the same result.
• There is also a point worth discussing together with this, that is #165, if that is a useful API modification and if both PRs are useful additions we can land together.

Let me know what you think @natecook1000 @timvermeulen @lorentey

Checklist

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

[LucianoPAlmeida]

A quick bench about difference in perf for a single all elements iteration just out of curiosity

var benchmark = Benchmark(title: "Rotated Algorithm")
let inputGenerator: (Int) -> [Int] = { size in
  return (0..<size).map { $0 }
}
benchmark.registerInputGenerator(inputGenerator)

benchmark.add(
  title: "In place rotation 25%",
  input: [Int].self
) { input in
  var mInput = input
  let count = mInput.count / 4 // 25% rotation
  return { timer in
    mInput.rotate(toStartAt: count)
    for value in mInput {
      blackHole(value)
    }
  }
}

benchmark.add(
  title: "Rotated View 25%",
  input: [Int].self
) { input in
  let count = input.count / 4 // 25% rotation
  return { timer in
    for value in input.rotated(toStartAt: count) {
      blackHole(value)
    }
  }
}

benchmark.main()