Rollup merge of #22729 - alexcrichton:ptr-stabilization, r=aturon

 Specifically, the following actions were takend:

* The `copy_memory` and `copy_nonoverlapping_memory` functions
  to drop the `_memory` suffix (as it's implied by the functionality). Both
  functions are now marked as `#[stable]`.
* The `set_memory` function was renamed to `write_bytes` and is now stable.
* The `zero_memory` function is now deprecated in favor of `write_bytes`
  directly.
* The `Unique` pointer type is now behind its own feature gate called `unique`
  to facilitate future stabilization.

[breaking-change]

diff --cc src/libcollections/slice.rs
Simple merge

diff --cc src/libcollections/string.rs
Simple merge

diff --cc src/libcollections/vec.rs
Simple merge

diff --cc src/libcollections/vec_deque.rs
Simple merge

diff --cc src/libstd/io/buffered.rs
Simple merge