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)
Right now the `MemoryMap` API exposes a number of platform-specific options such as `MapFd` which takes a `c_int` on unix and a `HANDLE` on windows. Other flags like `MapNonStandardFlags` are completely ignored on windows and only used on unix. Differences like this show that this API is not quite ready for prime time just yet.

As explained in [this comment](https://github.com/rust-lang/rfcs/pull/517#issuecomment-66868363) the movement into `os::platform` is not a declaration that this type is not portable. It is a declaration that the current implementation does not meet the [portability requirements](https://github.com/aturon/rfcs/blob/io-os-reform/text/0000-io-os-reform.md#what-cross-platform-means) of `std::io`. Once a proper API has been developed, we can stabilize both the `os::platform` and `std::io` versions.