Generalise range_stream to adapt arbitrary ranges to async-streams

The current implementation of `range_stream` produces a stream of integers in some specified range.

This was largely built just so we had some kind of stream that we could test against, but it would be much more useful to change this to take an arbitrary range (view/container) and allow construction of a stream that produces the elements of that range.

Existing uses of the `range_stream` could be changed to `range_stream(std::views::iota(min, max))`.
Or we could rename the existing stream `iota_stream` and then add a more general `range_stream`.