[Proposal] Add Interleave overloads.

[Orace]

Given an input of type IEnumerable<IEnumerable<T>> its hard to call Interleave on it:

Enumerable.Empty<T>.Interleave(input);
input.First().Interleave(input.Skip(1));

I propose to add an Interleave overload as a classic static method:

 public static IEnumerable<T> Interleave<T>(IEnumerable<IEnumerable<T>> otherSequences);

// usage
MoreEnumerable.Interleave(input);

Or as an extension method:

 public static IEnumerable<T> Interleave<T>(this IEnumerable<IEnumerable<T>> otherSequences);

// usage
input.Interleave();

We can also add an overload with the SelecMany pattern:

 public static IEnumerable<TResult> Interleave<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, IEnumerable<TResult>> selector);

// usage
new [] {"AAA", "BBB", "CCC"}.Interleave(e => e); // 'A', 'B', 'C', 'A', 'B', 'C', 'A', 'B', 'C'