Index struct needed by C# 8 range and index features

Index.cs

public readonly struct Index
{
  private readonly int _value;

  public int Value => _value < 0 ? ~_value : _value;
  public bool FromEnd => _value < 0;

  public Index(int value, bool fromEnd)
  {
    if (value < 0) throw new ArgumentException("Index must not be negative.", nameof(value));

    _value = fromEnd ? ~value : value;
  }

  public static implicit operator Index(int value)
    => new Index(value, fromEnd: false);
}