core: Finish stabilizing the mem module.

* All of the *_val functions have gone from #[unstable] to #[stable]
* The overwrite and zeroed functions have gone from #[unstable] to #[stable]
* The uninit function is now deprecated, replaced by its stable counterpart,
  uninitialized

[breaking-change]

src/libregex_macros/lib.rs

@@ -254,8 +254,8 @@ fn exec<'t>(which: ::regex::native::MatchKind, input: &'t str,
                 // The idea here is to avoid initializing threads that never
                 // need to be initialized, particularly for larger regexs with
                 // a lot of instructions.
-                queue: unsafe { ::std::mem::uninit() },
-                sparse: unsafe { ::std::mem::uninit() },
+                queue: unsafe { ::std::mem::uninitialized() },
+                sparse: unsafe { ::std::mem::uninitialized() },
                 size: 0,
             }
         }