Rollup merge of #100076 - tspiteri:const_slice_split_at, r=oli-obk

make slice::{split_at,split_at_unchecked} const functions

Now that `slice::from_raw_parts` is const in stable 1.64, it makes sense to have `split_at` const as well, otherwise unsafe code is required to achieve a const equivalent.

library/core/src/lib.rs

@@ -149,6 +149,7 @@
 #![feature(maybe_uninit_uninit_array)]
 #![feature(ptr_metadata)]
 #![feature(slice_ptr_get)]
+#![feature(slice_split_at_unchecked)]
 #![feature(str_internals)]
 #![feature(utf16_extra)]
 #![feature(utf16_extra_const)]