Generic IO and Windows IO were identical, since we don't do anything
windows specific (maybe when someone eventually wants to implement an IO
back-end for windows `IO_RING` API, we can bring it back :)
This does use the exact impl of `WindowsIO`, simply renaming it to
Generic IO.. because I don't think GenericIO had ever been used.
Reviewed-by: Pedro Muniz (@pedrocarlo)
Closes#2790