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)
Very rough idea: we might be able to define a wrapper struct `CancellableReader` etc. to support cancellation of I/O in the same way `BufferedReader` provides buffered I/O.

I haven't followed the discussion between @alexcrichton and @thestinger .  Here I can only emphasize we should honor the pay-as-you-go principle just like the C++ community does.  Convenient functions with overhead should be opt-in.