Combine Coroutines and Input Ranges for Dead-Simple D Iteration

[jerro]

> Compared to normal iteration schemes, yes.  It may be 
> comparable to opApply in terms of performance though.  If 
> you're curious, look at the Fiber context switching code 
> (starting at switchIn and switchOut).

It's much slower than opApply. This loop takes 70s on my machine:


foreach(el; visitor(Iterable(1000_000_000)))
     sum += 1;

And this one takes 2.7s:

auto iter(int n)
{
     struct I
     {
         int n;

         int opApply(int delegate(ref int) dg)
         {
             for(int i = 0; i < n; i++)
                 if(int r = dg(i))
                     return r;
             return 0;
         }
     }

     return I(n);
}

foreach(el; iter(1000_000_000))
     sum += 1;