library: Stabilize const_ptr_write

Const-stabilizes:
- `write`
- `write_bytes`
- `write_unaligned`

In the following paths:
- `core::ptr`
- `core::ptr::NonNull`
- pointer `<*mut T>`

Const-stabilizes the internal `core::intrinsics`:
- `write_bytes`
- `write_via_move`

library/core/src/lib.rs

@@ -137,7 +137,6 @@
 #![feature(const_pointer_is_aligned)]
 #![feature(const_ptr_is_null)]
 #![feature(const_ptr_sub_ptr)]
-#![feature(const_ptr_write)]
 #![feature(const_raw_ptr_comparison)]
 #![feature(const_replace)]
 #![feature(const_size_of_val)]