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)
Actually requesting only 10 bytes sounds different than what this function's name describes. For the "only read 10 bytes" case, I'd expect one would pass a `buffer.slice_to(10)` to read.