std: Stabilize some `ptr` functions

Specifically, the following actions were taken:

* 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.
* All type parameters now are `T: ?Sized` wherever possible and new clauses were
  added to the `offset` functions to require that the type is sized.

[breaking-change]

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