Const-stabilize Vec::new.

src/liballoc/vec.rs

@@ -314,10 +314,10 @@ impl<T> Vec<T> {
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_unstable(feature = "const_vec_new")]
+    #[cfg_attr(bootstrap, rustc_const_unstable(feature = "const_vec_new"))]
     pub const fn new() -> Vec<T> {
         Vec {
-            buf: RawVec::new(),
+            buf: RawVec::NEW,
             len: 0,
         }
     }