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)
@thestinger Why are you being so antagonistic? I'm not trying to "rewrite history". I've asked repeatedly for you to provide some source for your claim that there has been general consensus, and this is the first time you've actually provided *any* source whatsoever, and it doesn't even include the alleged rust-dev conversation.

Issue #6045 is a very general "don't use functions where methods are appropriate" issue. It's completely non-controversial and doesn't say anything in particular about the `raw` case. Issue #2868 is even less relevant, that's just "don't have functions that duplicate methods".

As for the rust-dev conversation, you still haven't actually linked it. If you want to use a rust-dev conversation to support your claim, you need to actually link it so we can read it.

Also, FWIW, 4 people does not a "general consensus" make.