Add carrying_add, borrowing_sub, widening_mul, carrying_mul methods to integers

library/core/src/lib.rs

@@ -76,6 +76,7 @@
 #![feature(const_alloc_layout)]
 #![feature(const_arguments_as_str)]
 #![feature(const_assert_type)]
+#![feature(const_bigint_helper_methods)]
 #![feature(const_caller_location)]
 #![feature(const_cell_into_inner)]
 #![feature(const_discriminant)]