Remove Io interface and use Io.Unix directly instead

[clecat]

As discussed a bit here, we should focus on making the codebase easier & remove some functors.
One of the way we can make it is by removing the Io functor which was used for a potential future implementation for mirage, but as of now, I think we should remove it and use directly Io.Unix (which is forced at some point anyway) and simply use Io.Unix directly.
This was first proposed by @Ngoguey42 who exposed several implications:

• Merging io_errors into errors
• Changing misc_error to unix_error
• Removing all functor arguments to: append_only_file, chunked_suffix, control_file, dispatcher, file_manager, mapping_file
  I think it would be a good start and would help making changes easier, and we could still functorize it back once the support for mirage is done.