Add MaybeUninit::new

Sometimes it *is* initialized!

src/libcore/lib.rs

@@ -82,6 +82,7 @@
 #![feature(const_fn)]
 #![feature(const_int_ops)]
 #![feature(const_fn_union)]
+#![feature(const_manually_drop_new)]
 #![feature(custom_attribute)]
 #![feature(doc_cfg)]
 #![feature(doc_spotlight)]