Stabilize const {slice,array}::from_mut

[ultrabear]

This PR stabilizes the following APIs as const stable as of rust 1.83:

// core::array
pub const fn from_mut<T>(s: &mut T) -> &mut [T; 1];

// core::slice
pub const fn from_mut<T>(s: &mut T) -> &mut [T];

This is made possible by const_mut_refs being stabilized (yay).

Tracking issue: #90206

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[cuviper]

I think this still needs a stabilization FCP.

r? libs-api

[ultrabear]

I think this still needs a stabilization FCP.

Yep, this was opened because the original issue already had an FCP for the half of it that got merged, and an FCP cant happen on the same issue twice (informed by Jubilee)

[ultrabear]

force pushed to squash what was 3 commits of failing bootstrap