Support deserializing, editing, and serializing with preserving Anchors(&) and Aliases(*)

[norio-nomura]

On current implementation, information about Anchors(&) and Aliases(*) are lost while compose().
e.g.:

import Foundation
import Yams

let node = try Yams.compose(yaml: """
aliases:
  - &steps-for-linux
    - checkout
    - run: swift test
version: 2
jobs:
  linux_swift_4.0.3:
    docker:
      - image: norionomura/swiftlint:swift-4.0.3
    steps: *steps-for-linux

  linux_swift_4.1.1:
    docker:
      - image: norionomura/swiftlint:swift-4.1.1
    steps: *steps-for-linux

""")

let string = try Yams.serialize(node: node!)
print(string)

output:

aliases:
- - checkout
  - run: swift test
version: 2
jobs:
  linux_swift_4.0.3:
    docker:
    - image: norionomura/swiftlint:swift-4.0.3
    steps:
    - checkout
    - run: swift test
  linux_swift_4.1.1:
    docker:
    - image: norionomura/swiftlint:swift-4.1.1
    steps:
    - checkout
    - run: swift test

To preserve Anchors and Aliases, it will need to introducing Node.alias(String).