Add const_slice_split_at_mut Feature gate.

library/core/src/lib.rs

@@ -137,6 +137,7 @@
 #![feature(const_size_of_val)]
 #![feature(const_slice_from_raw_parts_mut)]
 #![feature(const_slice_ptr_len)]
+#![feature(const_slice_split_at_mut)]
 #![feature(const_str_from_utf8_unchecked_mut)]
 #![feature(const_swap)]
 #![feature(const_trait_impl)]