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)
There are problems with using a `raw` module that are not addressed. There are modules are `btree` `treemap`, `trie` exposing more than one type and a `raw` module would fail to distinguish between these. A `raw` module also interacts very poorly with re-exports and adds verbosity to usage of the API. The motivation for making `unsafe` code uglier in these cases is not laid out.