core: Finish stabilizing the `mem` module.

* All of the *_val functions have gone from #[unstable] to #[stable]
* The overwrite and zeroed functions have gone from #[unstable] to #[stable]
* The uninit function is now deprecated, replaced by its stable counterpart,
  uninitialized

[breaking-change]
auto merge of #14392 : alexcrichton/rust/mem-updates, r=sfackler

* All of the *_val functions have gone from #[unstable] to #[stable]
* The overwrite and zeroed functions have gone from #[unstable] to #[stable]
* The uninit function is now deprecated, replaced by its stable counterpart,
  uninitialized

[breaking-change]