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)
These methods are occasionally very useful when you don't need to read the entire stream but only a few lines or characters. I have several of these in my code base. The supposed replacement
```rust
let line = r.lines().next().unwrap()
```
doesn't look really good.

Also, why `chars()` is on `Reader`? Doesn't reading characters require buffering?