Stabilize slice_as_chunks library feature

[scottmcm]

Draft as this needs #139163 to land first.

FCP: #74985 (comment)

Methods being stabilized are:

impl [T] {
    const fn as_chunks<const N: usize>(&self) -> (&[[T; N]], &[T]);
    const fn as_rchunks<const N: usize>(&self) -> (&[T], &[[T; N]]);
    const unsafe fn as_chunks_unchecked<const N: usize>(&self) -> &[[T; N]];
    const fn as_chunks_mut<const N: usize>(&mut self) -> (&mut [[T; N]], &mut [T]);
    const fn as_rchunks_mut<const N: usize>(&mut self) -> (&mut [T], &mut [[T; N]]);
    const unsafe fn as_chunks_unchecked_mut<const N: usize>(&mut self) -> &mut [[T; N]];
}

(FCP's not done quite yet, but will in another day if I'm counting right.) FCP Complete: #74985 (comment)

[rustbot]

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[rustbot]

The Miri subtree was changed

cc @rust-lang/miri

[scottmcm]

@rust-lang/libs-api I wanted to drop you a ping because there has been a reasonable amount of discussion during the FCP, so if you want me to hold off this for a bit to give you discussion time, let me know.

(I haven't seen any indication from comments that you've changed your minds, but it might just not have made it into the thread.)

[BurntSushi]

Aye yeah, I'm still good with this. But given the discussion, holding for a second from another libs-api member seems prudent.

[dtolnay]

LGTM as well. Thank you. We have talked about this feature across at least a half dozen library API team meetings and not diverged from favoring this design.

[dtolnay]

In the section above (which I cannot comment because it is too far from the diff) we need to strike "This check will most probably get changed to a compile time error before this method gets stabilized."

[scottmcm]

Oops, that had even been mentioned in #74985 (comment) and I forgot.

Did a pass over the docs to fix that bit, elaborate on the resulting lengths, and also add a cross-references between these methods and as_flattened(_mut).

@rustbot ready