MaybeUninit array cleanup
* Links `MaybeUninit::uninit_array` to meta-tracking issue
* Links `MaybeUninit::array_assume_init` to meta-tracking issue
* Unstably constifies `MaybeUninit::array_assume_init`
Another thing worth mentioning: this splits the const feature flag for `maybe_uninit_uninit_array` into `const_maybe_uninit_uninit_array` to avoid weird cases where only one gets stabilised.
Note that it may be desired to keep the `array_assume_init` method linked to its dedicated issue, but at least for now, I decided to link to the meta-tracking issue so that all of the methods lead users to the same place. But I can revert that bit if desired.
The meta-tracking issue that I filed is #96097.
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