Add deprecated re-export of ImageReader as Reader

[ripytide]

As requested in #2243, some module renaming was required to ensure the current io can become private while still re-exporting the deprecated Reader type synonym from a top-level module called io.

[fintelia]

Looking more closely, io::Limits and io::LimitSupport will also need to be re-exported to avoid it being a breaking change

[ripytide]

I've added re-exports for them too.

[fintelia]

Thanks!