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 useful for composition where later methods end up consuming `self`. For example:

```rust
iter.by_ref().map(...);
// vs
(&mut iter).map(...);
```

I believe we do actually have an implementation of `Writer` for `&mut W: Writer`, but the `by_ref` method just helps with chaining. (iterators have this as well).