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)
@alexcrichton The current implementation of `BufferedReader` simply calls `read()` with its entire internal buffer.  Then it should suffice to have a simpler convenience method like the below one in `Reader` because it will be "inherited" by `BufferedReader`:

```rust
fn read_exact(&mut self, buf: &mut [u8]) -> NonatomicResult<(), uint, Err> {
    let mut read_so_far = 0;
    while read_so_far < buf.len() {
        match self.read(buf.slice_from_mut(read_so_far)) {
            Ok(n) => read_so_far += n,
            Err(e) => return NonatomicResult(read_so_far, e)
        }
    }
    Ok(())
}
```

(cf. PR rust-lang/rust#18059 )