Make checked arithmetic besides division const

Co-Authored-By: 9999years <rbt@sent.as>

src/libcore/lib.rs

@@ -72,6 +72,7 @@
 #![feature(concat_idents)]
 #![feature(const_alloc_layout)]
 #![feature(const_if_match)]
+#![feature(const_int_checked)]
 #![feature(const_int_euclidean)]
 #![feature(const_panic)]
 #![feature(const_fn_union)]