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)
> Using WTF-8 is not about saving memory. 

I didn't claim the intention behind WTF-8 must have been saving memory.  I claimed the only achievement of it would be saving memory for Americans/British//Europeans at the cost of consuming everyone's CPU more.

> FUD. next_surrogate() is only using during WTF-8 to UTF-8 conversions, 

`fmt::Show:show()` on `Wtf8` calls `next_surrogate()` at least once (I'd colloquially say it makes `show()` an "*O(2n)*" operation.)  It is such a basic operation on a string-like object.  You can argue it is one form of "WTF-8 to UTF-8 conversion," but it is misleading.