RFC: io and os reform

This RFC proposes a significant redesign of the `std::io` and `std::os` modules
in preparation for API stabilization. The specific problems addressed by the
redesign are given in the Problems section below, and the key ideas of the
design are given in Vision for IO.

[Rendered](https://github.com/aturon/rfcs/blob/io-os-reform/text/0000-io-os-reform.md)
It has a name conflict with the real meaning of attribute in the context of file systems. It's not an OS specific name. It's even used on DOS.

https://en.wikipedia.org/wiki/File_attribute

> Attributes are considered distinct from other metadata, such as dates and times, filename extensions or file system permissions.

There are also extended attributes, which is a side table of key values that the filesystem doesn't interpret itself.

 https://en.wikipedia.org/wiki/Extended_file_attributes