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)
Yeah they're the analog of the methods in `os` today, and they're precisely dealing with `:` vs `;` as well as some weird quoting rules on windows. I believe that the [quoting problem](https://github.com/rust-lang/rust/blob/master/src/libstd/os.rs#L513-L548) on windows is why we're not just exposing a separator to use with other methods (sadly).

I do, however, prefer the name `connect` to `join` slightly!