Rollup merge of #131274 - workingjubilee:stabilize-the-one-that-got-a…

…way, r=scottmcm

library: Const-stabilize `MaybeUninit::assume_init_mut`

FCP completed in rust-lang/rust#86722 (comment)

Also moves const-ness of an unstable fn under the `maybe_uninit_slice` gate, Cc rust-lang/rust#63569