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)
You only have to look in the git history to see that we migrated away from `raw` modules *as part of replacing free functions with methods*. It wasn't treated as a special case because no one thought `unsafe` was inadequate. I was only the listing some of the people involved in implementing the consensus, not everyone involved in the discussion. I'm not going to waste time digging up all of the mailing list threads about functions vs. methods and UFCS to satisfy you, since you'll just come up with something else to nitpick.