p1664 - reconstructible_range and ranges::reconstruct Extension Point

The same paper as #20, but it needs to be redone to stick it to the fact that `string_view` is the only string view type it works with, and that it'll exclude Niall Douglass's `io_span` and similar.

This is because we hard-coded `string_view`, `span`, and other types directly into the "reduce template bloat" paper from Hannes Hauswedell. This was a compromise, and absolutely the wrong decision.