Auto merge of rust-lang#124498 - tgross35:stabilize-non_null_convenience, r=jhpratt

Stabilize `non_null_convenience`

Fully stabilize the following API, including const where applicable:

```rust
impl <T> NonNull<T> {
    pub const unsafe fn offset(self, count: isize) -> Self;
    pub const unsafe fn add(self, count: usize) -> Self;
    pub const unsafe fn sub(self, count: usize) -> Self;
    pub const unsafe fn offset_from(self, origin: NonNull<T>) -> isize;
    pub const unsafe fn read(self) -> T;
    pub unsafe fn read_volatile(self) -> T;
    pub const unsafe fn read_unaligned(self) -> T;
    pub unsafe fn write_volatile(self, val: T);
    pub unsafe fn replace(self, src: T) -> T;
}

impl<T: ?Sized> NonNull<T> {
    pub const unsafe fn byte_offset(self, count: isize) -> Self;
    pub const unsafe fn byte_add(self, count: usize) -> Self;
    pub const unsafe fn byte_sub(self, count: usize) -> Self;
    pub const unsafe fn byte_offset_from<U: ?Sized>(self, origin: NonNull<U>) -> isize;
    pub unsafe fn drop_in_place(self);
}
```

Stabilize the following without const:

```rust
impl <T> NonNull<T> {
    // const under `const_intrinsic_copy`
    pub const unsafe fn copy_to(self, dest: NonNull<T>, count: usize);
    pub const unsafe fn copy_to_nonoverlapping(self, dest: NonNull<T>, count: usize);
    pub const unsafe fn copy_from(self, src: NonNull<T>, count: usize);
    pub const unsafe fn copy_from_nonoverlapping(self, src: NonNull<T>, count: usize);

    // const under `const_ptr_write`
    pub const unsafe fn write(self, val: T);
    pub const unsafe fn write_bytes(self, val: u8, count: usize);
    pub const unsafe fn write_unaligned(self, val: T);

    // const under `const_swap`
    pub const unsafe fn swap(self, with: NonNull<T>);

    // const under `const_align_offset`
    pub const fn align_offset(self, align: usize) -> usize;

    // const under `const_pointer_is_aligned`
    pub const fn is_aligned(self) -> bool;
}
```

Left the following unstable:

```rust
impl <T> NonNull<T> {
    // moved gate to `ptr_sub_ptr`
    pub const unsafe fn sub_ptr(self, subtracted: NonNull<T>) -> usize;
}

impl <T: ?Sized> NonNull<T> {
    // moved gate to `pointer_is_aligned_to`
    pub const fn is_aligned_to(self, align: usize) -> bool;
}
```

Fixes: rust-lang#117691

Author: bors

library/alloc/src/lib.rs

@@ -137,7 +137,6 @@
 #![feature(maybe_uninit_slice)]
 #![feature(maybe_uninit_uninit_array)]
 #![feature(maybe_uninit_uninit_array_transpose)]
-#![feature(non_null_convenience)]
 #![feature(panic_internals)]
 #![feature(pattern)]
 #![feature(ptr_internals)]

library/core/src/lib.rs

@@ -180,7 +180,6 @@
 #![feature(isqrt)]
 #![feature(link_cfg)]
 #![feature(maybe_uninit_uninit_array)]
-#![feature(non_null_convenience)]
 #![feature(offset_of_enum)]
 #![feature(offset_of_nested)]
 #![feature(panic_internals)]