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)
Raw `read` and `write` calls are *always* thread-safe. It is possible for large writes to overlap but it is still thread safe. File descriptors and Windows handles are already reference counting handles, so there's not a good reason to do an extra layer of dynamic memory allocation if it's not being explicitly asked for.