Rollup merge of #126711 - GKFX:const-option-as-slice, r=oli-obk

Make Option::as_[mut_]slice const

These two functions can both be made `const`. I have added them to the `const_option_ext` feature, #91930. I don't believe there is anything blocking stabilization of `as_slice`, but `as_mut_slice` contains mutable references so depends on `const_mut_refs`.

Author: matthiaskrgr