Stabilize some `MaybeUninit` behavior as const
This stabilizes the `MaybeUninit::as_ptr`, `MaybeUninit::assume_init`, and `MaybeUninit::assume_init_ref` as `const fn`. `MaybeUninit::as_mut_ptr` has been moved to a new flag: `const_maybe_uninit_as_mut_ptr`, which is blocked on #57349. `MaybeUninit::slice_assume_init_ref` can be `const fn` when the method is stabilized in general.
The relevant intrinsic has been stabilized as `const` as well, though this isn't user-visible. Due to the seemingly unrelated feature name I performed `rg const_assert_type` and found no other instances of it being used.
r? `@oli-obk`
`@rustbot` label: +A-const-fn +S-waiting-on-review +T-libs-api
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