Range struct needed by C# 8 range features

Range.cs

public readonly struct Range
{
  public Index Start { get; }
  public Index End { get; }

  private Range(Index start, Index end)
    => (Start, End) = (start, end);

  public static Range Create(Index start, Index end) => new Range(start, end);
  public static Range FromStart(Index start) => new Range(start, new Index(0, fromEnd: true));
  public static Range ToEnd(Index end) => new Range(new Index(0, fromEnd: false), end);
  public static Range All() => new Range(new Index(0, fromEnd: false), new Index(0, fromEnd: true));
}