turn rustc_box into an intrinsic

library/alloc/src/lib.rs

@@ -168,6 +168,7 @@
 #![feature(dropck_eyepatch)]
 #![feature(fundamental)]
 #![feature(hashmap_internals)]
+#![feature(intrinsics)]
 #![feature(lang_items)]
 #![feature(min_specialization)]
 #![feature(multiple_supertrait_upcastable)]