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)
A string not being UTF-8 or a file not existing aren't any more of an error than a key not existing in a map. Attempting to open a file or parse text is also a way to discover if what you were looking for was there, just like a map lookup. There are few remaining use cases for `Option` if it's not meant to be used this way... any missing value can be considered an error, just like a missing file / whatever.