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)
It's less of a memory safety issue and more of an assumption that code can use to avoid weird loops like this: https://github.com/rust-lang/rust/blob/master/src/libstd/io/mod.rs#L512-L517. It could be replaced with just returning an error immediately or something like that. The underlying C-level interfaces can never return 0 byte reads anyway, since 0 means EOF.