Make the Metadata struct Clone

This commit adds #[derive(Clone)] to std::fs::Metadata, making that struct
cloneable. Although the exact contents of that struct differ between OSes,
they all have it contain only value types, meaning that the data can be re-used without repercussions.

It also adds #[derive(Clone)] to every type used by that struct across all
OSes, including the various Unix `stat` structs and Windows's
`WIN32_FILE_ATTRIBUTE_DATA`.

src/libstd/fs.rs

@@ -60,6 +60,7 @@ pub struct File {
 /// represents known metadata about a file such as its permissions, size,
 /// modification times, etc.
 #[stable(feature = "rust1", since = "1.0.0")]
+#[derive(Clone)]
 pub struct Metadata(fs_imp::FileAttr);
 
 /// Iterator over the entries in a directory.