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)
I'm a bit confused here. `Vec::from_raw_parts` seems to be covered under the convention for `raw` submodules, as it is constructing a `Vec` from the low-level representation (pointer, length, capacity). If it's put in the `raw` submodule then it shouldn't have `raw` in the name, because that's unnecessarily verbose (`raw::from_raw_parts`?),