Implements "random" properties for collections and ranges

Collection+random.swift

import Darwin

extension RandomAccessCollection {
  // Fails if collection is larger than UInt32.max
  var randomIndex: Index {
    guard !isEmpty else { return endIndex }
    let fullDistance = UInt32(distance(from: startIndex, to: endIndex))
    let randomDistance = IndexDistance(arc4random_uniform(fullDistance))
    return index(startIndex, offsetBy: randomDistance)
  }

  var random: Element? {
    let index = randomIndex
    if index == endIndex {
      return nil
    } else {
      return self[index]
    }
  }
}

// Preconditions: these should always hold
[].randomIndex == 0 // true
[1].randomIndex == 0 // true
[1,2][1...].randomIndex == 1 // true
[1,2][2...].randomIndex == 2 // true
(0 ..< UInt32.max).randomIndex != Int(UInt32.max) // true
(0 ... UInt32.max).count > Int(UInt32.max)
//(0 ... UInt32.max).randomIndex // Failure: not enough bits for signed value

// Example usage: what should I eat tonight?
print(Array("🌭🍕🌯🍗🍝🍜🍣🍛🍺🍳").random)

Requires Swift 4

This is now redundant, thanks to the new random APIs in Swift 4.2!