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)
Did you mean `-> Vec<u16>` here? The only way to have both `-> &[u16]` *and* `pub fn as_str(&self) -> Option<&str>;` is to keep both representations around. Is it worth the memory cost? I suppose the WTF-8 representation could then be initialized lazily, skipping conversions entirely for e.g. opening a `Path` from `readdir` on Windows. Or maybe that wouldn’t work (could lazy initialization be done at all in methods that take `&self` and not `&mut self`?)