RFC on range design for D2

[BCS]

Reply to Andrei,

> Hello,
> 
> Walter, Bartosz and myself have been hard at work trying to find the
> right abstraction for iteration. That abstraction would replace the
> infamous opApply and would allow for external iteration, thus paving
> the way to implementing real generic algorithms.

First of all, I /Like/ opApply. I know there are issues with it so I'd rather 
see it supplemented rather than replaced.