stabilize const_intrinsic_copy

[RalfJung]

Fixes #80697

This stabilizes

mod ptr {
    pub const unsafe fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize);
    pub const unsafe fn copy<T>(src: *const T, dst: *mut T, count: usize);
}

impl *const T {
    pub const unsafe fn copy_to(self, dest: *mut T, count: usize);
    pub const unsafe fn copy_to_nonoverlapping(self, dest: *mut T, count: usize);
}

impl *mut T {
    pub const unsafe fn copy_to(self, dest: *mut T, count: usize);
    pub const unsafe fn copy_to_nonoverlapping(self, dest: *mut T, count: usize);

    pub const unsafe fn copy_from(self, src: *const T, count: usize);
    pub const unsafe fn copy_from_nonoverlapping(self, src: *const T, count: usize);
}

impl <T> NonNull<T> {
    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);
}

In particular, this reverts #117905, which reverted #97276.

The NonNull methods are not listed in the tracking issue, they were added to this feature gate in #124498. The existing FCP does not cover them. They are however entirely identical to the *mut methods and already stable outside const. @rust-lang/libs-api please let me know if FCP will be required for the NonNull methods.

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Portable SIMD is developed in its own repository. If possible, consider making this change to rust-lang/portable-simd instead.

cc @calebzulawski, @programmerjake

Thank you!

@bors r+

📌 Commit 2787179 has been approved by dtolnay

It is now in the queue for this repository.