Make saturating_mul a const fn

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

src/libcore/lib.rs

@@ -75,6 +75,7 @@
 #![feature(const_int_checked)]
 #![feature(const_int_euclidean)]
 #![feature(const_int_overflowing)]
+#![feature(const_int_saturating)]
 #![feature(const_panic)]
 #![feature(const_fn_union)]
 #![feature(const_generics)]