Input/Output refactoring

CleanUpPolicy.swift

  protocol CleanUpPolicy {
    func itemsToRemove(from items: Set<OnDiskCache.Item>) -> Set<OnDiskCache.Item>
  }

  struct MaxSizeCachePolicy: CleanUpPolicy {
    let maxSize: Int
    
    func itemsToRemove(from items: Set<OnDiskCache.Item>) -> Set<OnDiskCache.Item> {
      var itemToRemove = Set<OnDiskCache.Item>()
      let sortedItems = items.sorted { $0.age < $1.age }
      var cumulativeSize = 0
      for item in sortedItems {
        cumulativeSize += item.size
        if cumulativeSize > maxSize {
         itemToRemove.insert(item)
        }
    }
}