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)
We must rename either of two `BufferedReader`s.

```rust
trait X { }
struct X;

fn main() {
}
```

- http://is.gd/qbiquo [playpen]

```
<anon>:2:1: 2:10 error: duplicate definition of type or module `X`
<anon>:2 struct X;
         ^~~~~~~~~
<anon>:1:1: 1:12 note: first definition of type or module `X` here
<anon>:1 trait X { }
         ^~~~~~~~~~~
error: aborting due to previous error
```