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)
The idea of `carbon_copy()` fits more with a separate convenience wrapper similar to `BufferedReader` / `Writer`:

```rust
let mut f = File::create(...);
let mut stdout = io::stdio::stdout();
let mut tee = TeeWriter(f, stdout);
tee.write_all(b"Hello world!\n");
```

Anyways, the semantics of partial failure need to be discussed in detail.