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)
In the case of a buffered reader, I would consider it a performance optimization in terms of the number of reads of the underlying stream. The buffered reader can pass down a very large buffer but only request that a tiny part gets filled, and if more than that is filled in then it results in, for example, fewer syscalls (in theory).