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)
This is further from the API provided by the OS. It's the same mistake of building an abstracting that doesn't actually correspond to how things work. If the underlying OS design is *portable* and *safe* (which it is) then it should be exposed directly. It might make sense to abstract over it another way, but doing so inside the standard library is dubious.