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)
Same comment as for `OsStrExt::to_wide`, this seems to assume that the internal representation is `Vec<u16>`. If it’s WTF-8, these methods could have less restrictive ownership constraints: `from_slice(&[u16]) -> Self` and `to_vec(&self) -> Vec<u16>`.

Also, the names maybe should be consistent (`vec` v.s. `wide`).