Turn .opApply into ranges

[Yuxuan Shui via Digitalmars-d]

I wondered if I can turn struct that defines opApply into ranges.

And it turns out to be surprisingly easy:

https://gist.github.com/yshui/716cfe987c89997760cabc2c951ca430

Maybe we can phase out opApply support in foreach? ;)

BTW, is there a way to get the "element type" from .opApply?