Add unstable-const-fn feature to make new() functions const.

src/maybe_uninit.rs

@@ -16,7 +16,7 @@ impl<T> Clone for MaybeUninit<T>
 
 impl<T> MaybeUninit<T> {
     /// Create a new MaybeUninit with uninitialized interior
-    pub unsafe fn uninitialized() -> Self {
+    pub const unsafe fn uninitialized() -> Self {
         MaybeUninit { inner: StdMaybeUninit::uninit() }
     }