Rename Reader to ImageReader

[ripytide]

I license past and future contributions under the dual MIT/Apache-2.0 license,
allowing licensees to choose either at their option.

Following the discussion from #1327

[fintelia]

Created #2245 to track issues like this that'll need a breaking release

[kornelski]

I think it'd be helpful to create a pub use under old name with a deprecated attribute. It makes migration less painful.

[fintelia]

I'm not sure you can place a deprecated attribute on a re-export.

But in either case, in the future @kornelski let's avoid merging breaking changes directly to the main branch. I'd prefer to keep the main branch in a state where it is always possible to make a patch release if needed. We can discuss major release strategy more in the linked issue, but I wasn't planning to start it for another 6+ months.

[kornelski]

Deprecated attribute works fine on re-exports and correctly fires only when the reexported name is used.

The attribute needs to be on pub type. Still doable.

[fintelia]

@ripytide could you open a PR with the necessary re-export?

I'd also like to give @HeroicKatora a chance to weigh in. They had some concerns with the renaming before, and while they've been less active recently they are still a maintainer.

[HeroicKatora]

The previous design is to large parts modelled around the standard library, that is small modules and avoid prefix duplication (the library is image, it shouldn't preface everything with another Image*). That decision was before experience of wgpu has demonstrated, to me but also in general, that the inverse is also possible and a large top level is still a workable library usage experience. I'd prefer to keep the io module itself for uncommon types related specifically to it but I don't have any deeper concerns about the renaming.