/// <summary>
/// Rate limits a collection mapping.
/// </summary>
/// <typeparam name="T"></typeparam>
/// <typeparam name="U"></typeparam>
/// <param name="arr"></param>
/// <param name="limiter"></param>
/// <param name="f"></param>
/// <returns></returns>
public static Task <U[]> Select <T, U>(
this IEnumerable <T> arr,
ILimiter limiter,
Func <T, int, Task <U> > f
) => arr.Select((x, i) => limiter.Run(() => f(x, i))).WhenAll();
