Cleanup and stability for core::slice

This implements some of the recommendations from https://github.com/rust-lang/meeting-minutes/blob/master/Meeting-API-review-2014-08-06.md.

Explanation in commits.
The docstring for this method `unsafe_ref` that you didn't touch, and the identical docstring for the replacement `unsafe_get` It would be nice for the summary line to instead indicate that it's deprecated (or just delete the docstring altogether, since people shouldn't be using it anyway).