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)
Is the name `tee` really platform specific? More generally: does anything but unix have a concept that maps to this? If some people already know what `tee` means, isn't that better than no one knowing exactly what `carbon_copy` implies?