RFC: conventions for placement of unsafe APIs

This is a *conventions RFC* for settling the location of `unsafe` APIs relative
to the types they work with, as well as the use of `raw` submodules.

The brief summary is:

* Unsafe APIs should be made into methods or static functions in the same cases
  that safe APIs would be.

* `raw` submodules should be used only to provide APIs directly on low-level
  representations.

[Rendered](https://github.com/aturon/rfcs/blob/unsafe-api-location/active/0000-unsafe-api-location.md)
The `raw` modules that exist in the standard library were mostly recent additions without consensus based on the same push as this RFC. The previous consensus was to turn everything into methods, and only a few old `raw::` functions remained because no one had taken the time to port away from them yet.