std.format add dash separators to large numbers

[bearophile]

Jouko Koski:

> While 1_000_000 may be the right thing in program code, the outside world 
> would probably like to have numbers printed out according to their locale 
> conventions. This is even more important with floating point or monetary 
> values.
> 
> So before just adding some formatter to Phobos, I think that such 
> formatter's use cases should be considered quite carefully.

You are right, of course. On the other hand I don't think there are locales where the right way to print number is with underscores. But often I do want to print large numbers with underscores every three digits. So maybe your idea doesn't help me much.

Bye,
bearophile