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 `_pipe` function just calls `CreatePipe` so if the goal is avoiding the CRT you should just migrate to a direct call. It's no different than how Rust will have to move from calling `malloc` and `_aligned_malloc` to `HeapAlloc` + manually implemented alignment support.