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)
There are already in-memory endian conversion functions. It could just define a way to go from `[u8, ..n]` to the integer with width `n`. You read into the buffer and then convert. If no conversion is required, it will be a no-op and can likely be optimized out.