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)
I wonder if `io::stdio::stdout()` etc. should support customizing what it returns.  For example Python supports assignment to `sys.stdout`.  For example this `colorama` library wraps `sys.stdout` with its own adapter to interpret ANSI color escape sequences for Windows `cmd.exe` which doesn't understand them.

- https://github.com/tartley/colorama/blob/master/colorama/initialise.py

(This is only a nice-to-have feature.)