fs_utf8: Add as_cap_std() (#357)

I'm looking at adding support in https://github.com/coreos/cap-std-ext/
for `fs_utf8`, and doing so really wants access to the currently
internal `cap_std` member in order to share code - in the
same way basically all the implementation functions here
just delgate to the underlying `Dir`.

I changed one usage to call this helper.

Signed-off-by: Colin Walters <[email protected]>

cap-std/src/fs_utf8/dir.rs

@@ -54,14 +54,22 @@ impl Dir {
         Self { cap_std }
     }
 
+    /// Return a view of this directory as a [`cap_std::fs::Dir`]. This
+    /// is often useful in order to write shared functions which can operate
+    /// on either type.
+    #[inline]
+    pub fn as_cap_std(&self) -> &crate::fs::Dir {
+        &self.cap_std
+    }
+
     /// Attempts to open a file in read-only mode.
     ///
     /// This corresponds to [`std::fs::File::open`], but only accesses paths
     /// relative to `self`.
     #[inline]
     pub fn open<P: AsRef<Utf8Path>>(&self, path: P) -> io::Result<File> {
         let path = from_utf8(path.as_ref())?;
-        self.cap_std.open(path).map(File::from_cap_std)
+        self.as_cap_std().open(path).map(File::from_cap_std)
     }
 
     /// Opens a file at `path` with the options specified by `options`.