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)
> FromEnd(u64) shoud be FromEnd(i64)

If `FromEnd(10)` would seeks to 10 bytes before the EOF then `FromEnd(-10)` – 10 characters *after* the EOF. In what case seeking past EOF is valid?