Stabilize non_null_convenience

Fully stabilize the following API, including const where applicable:

    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:

    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:

    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> {
        pub const fn is_aligned_to(self, align: usize) -> bool;
    }

Fixes: #117691

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)]