Add Step extension

[atifaziz]

This PR adds an extension that iterates or steps through a sequence based on another sequence of integral steps to take between elements. In other words, an integral sequence of steps drives when the iterator of the first sequence is moved. An example below shows how this could be used to generate a random walk through a sequence:

var randomWalk =
    MoreEnumerable.Unfold(new Random(), r => (Random: r, Step: r.Next(2)), _ => true, s => s.Random, s => s.Step);
var result =
    from e in Enumerable.Range(1, 10).Step(randomWalk)
    select $"{(e.Step > 0 ? "Moved" : "Still")}: {e.Item}";
foreach (var s in result)
    Console.WriteLine(s.ToString());

Sample output:

Moved: 1
Moved: 2
Still: 2
Moved: 3
Moved: 4
Moved: 5
Still: 5
Moved: 6
Still: 6
Moved: 7
Still: 7
Moved: 8
Moved: 9
Still: 9
Moved: 10