Rollup merge of #130762 - RalfJung:const_intrinsic_copy, r=dtolnay

stabilize const_intrinsic_copy

Fixes rust-lang/rust#80697

This stabilizes
```rust
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 rust-lang/rust#117905, which reverted rust-lang/rust#97276.

The `NonNull` methods are not listed in the tracking issue, they were added to this feature gate in rust-lang/rust#124498. The existing [FCP](rust-lang/rust#80697 (comment)) 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.