Consider making the IterBuffer construction public

[KaiserKarel]

Right now, customizing parameters for more complex data fetching (offsets etc) isn't possible, due to the use of pub(crate). Ideally we'd add ways to customize these data fetches, but at the minimum allowing the library user to construct the IterBuffer will allow them to do whatever they could do with a more structured API surface as well.

[Lonami]

Yeah utilities to better work with raw API would be nice. That said, IterBuffer should probably first be fixed to use proper Streams.