Forward ranges in Phobos v2

[Paul Backus]

On Wednesday, 3 November 2021 at 00:01:33 UTC, H. S. Teoh wrote:
> The problem with manually-added tags of this sort is that 
> people forget to do it, and that leads to trouble.  Preferably, 
> it should be something already implicit in the range type 
> itself, that does not require additional effort to tag.
>
> I'm kinda toying with the idea of struct == forward range, 
> class == input range: the difference is inherent in the type 
> itself and requires no further effort beyond the decision to 
> use a by-value type vs. a by-reference type, which coincides 
> with the decision to make something an input range or a forward 
> range.

Having input ranges implement `next` and forward ranges implement 
`head` and `tail` would also make them easy to distinguish.