RFC: Raw Reform

* Introduce RawSlice and RawMutSlice extension traits to be implemented on
`*const [T]` and `*mut [T]` that provide parts of the slice and ptr API to better bridge the
gap between the two.

* Provide unsafe versions of the slicing and indexing operators on raw slices to provide ergonomic
unchecked slice manipulation.

* Provide unsafe versions of the addition, subtraction, and indexing operators on raw ptrs to
provide more ergonomic ptr manipulation.

* Deprecate some of the free functions in `ptr`, and duplicate the rest as convenience methods on
the RawPtr extension traits.

* Deprecate *all* `unsafe` methods on slices and `slice::raw` in favour of raw slices.

* Add conversion methods to the three families of extension traits for conveniently moving between
them.

[Rendered](https://github.com/Gankro/rfcs/blob/raw-reform/active/0000-raw-reform.md)
That's really good to know. @thestinger suggested to me on IRC that pointer arithmetic was highly desirable. If I understand the history correctly, it used to be there, but had to be removed because it had to be an Unsafe operation, and Add and Sub are safe. If that's true, then that implies the only reason it was removed was purely technical, and not philosophical.

Personally, I don't have a very strong opinion. `offset` is easier to chain, which I prefer. However it does make porting tricky C/C++ code easier.