Backward compatibility for `compactMap` in Swift 4.0.

shim-swift-4-1.swift

#if swift(>=4.1)
#else
extension Collection {
  func compactMap<ElementOfResult>(
    _ transform: (Element) throws -> ElementOfResult?
  ) rethrows -> [ElementOfResult] {
    return try flatMap(transform)
  }
}
#endif

shim-swift-4-2.swift

#if swift(>=4.1)
#elseif swift(>=4.2)
#warning("Remove this once we've updated.")
#else
extension Collection {
  func compactMap<ElementOfResult>(
    _ transform: (Element) throws -> ElementOfResult?
  ) rethrows -> [ElementOfResult] {
    return try flatMap(transform)
  }
}
#endif