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)
The one potential concern I have with per-object buffering is that caller's that don't realize that that's the behavior will be accidentally performing tons of allocation and deallocation as they repeatedly create transient `stdout` readers. It does solve the flush on exit problem, though, without requiring weird atexit handlers or anything like that.