Stabilize const slice::split_at

This stabilizes the use of the following method in const context:

    impl<T> [T] {
        pub const fn split_at(&self, mid: usize) -> (&[T], &[T]);
    }

Author: tspiteri

library/core/src/slice/mod.rs

@@ -1596,7 +1596,8 @@ impl<T> [T] {
     /// }
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_unstable(feature = "const_slice_split_at_not_mut", issue = "101158")]
+    #[rustc_const_stable(feature = "const_slice_split_at_not_mut", since = "CURRENT_RUSTC_VERSION")]
+    #[rustc_allow_const_fn_unstable(slice_split_at_unchecked)]
     #[inline]
     #[track_caller]
     #[must_use]