Stabilize const_slice_flatten

[DaniPopes]

Const-stabilizes slice::as_flattened{,_mut}:

// core::slice
impl<T, const N: usize> [[T; N]] {
    pub const fn as_flattened(&self) -> &[T];
    pub const fn as_flattened_mut(&mut self) -> &mut [T];
}

Tracking issue: #95629

Requires separate FCP, as per #95629 (comment).

Closes #95629.

@rustbot modify labels: +T-libs-api

Happy new year!

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[DaniPopes]

@rustbot label -T-libs +needs-fcp

[DaniPopes]

r? libs-api

[Amanieu]

These were previously stabilized but left as const-unstable because they took references, but this is no longer a blocker now that #129195 and #134927 are merged.

@rfcbot merge

[rfcbot]

Team member @Amanieu has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se

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