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)
Creating an replacement will involve copying a bunch of the methods defined on `Reader` and `Writer` into a new trait, impling them for `Reader` and `Writer` and pushing it to crates.io. That does not seem like some kind of nightmare scenario to me.