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)
Should this be `-> Result<&str, ()>`?

Or as a larger point (perhaps out of scope for this RFC), should `Option<T>` return types be `Result<T, ()>` instead when `None` kind of represents an error, in order to interoperate with `try!` and other error-handling infrastructure we might add?