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 keep asserting that this makes the API harder to use, but that's extremely subjective. I've always found the `raw` modules in `str`/`slice`/`string`/`vec` make the API much *easier* to use, because any time I need to construct a value from raw pointers, I know precisely where to look.