rollup merge of #19902: alexcrichton/second-pass-mem

This commit stabilizes the `mem` and `default` modules of std.

src/libstd/io/mod.rs

@@ -1911,7 +1911,9 @@ bitflags! {
 }
 
 
+#[stable]
 impl Default for FilePermission {
+    #[stable]
     #[inline]
     fn default() -> FilePermission { FilePermission::empty() }
 }