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 my micro-benchmarking, I found `BufWriter` to be 2+ times the speed of `MemWriter`. For the life of me, I couldn't figure out how to get the capacity check faster than what we have right now. While generally I would tell people to just go for `MemWriter`, something like `BufWriter` would be really helpful for doing high performance byte serialization when you statically the size of the output.