fs_utf8/Dir: impl AsRef<crate::fs::Dir> (#364)

This allows an API like:
```
fn foo(d: impl AsRef<Dir>) {
    ...
}
```

to accept either a `Dir` or a `DirUtf8`. I don't know that
I'll do this much, but I think it can make sense.

cap-std/src/fs_utf8/dir.rs

@@ -94,7 +94,7 @@ impl Dir {
     #[inline]
     pub fn open_dir<P: AsRef<Utf8Path>>(&self, path: P) -> io::Result<Self> {
         let path = from_utf8(path.as_ref())?;
-        self.cap_std.open_dir(path).map(Self::from_cap_std)
+        self.as_ref().open_dir(path).map(Self::from_cap_std)
     }
 
     /// Creates a new, empty directory at the provided path.
@@ -838,3 +838,9 @@ impl fmt::Debug for Dir {
         self.cap_std.fmt(f)
     }
 }
+
+impl AsRef<crate::fs::Dir> for Dir {
+    fn as_ref(&self) -> &crate::fs::Dir {
+        self.as_cap_std()
+    }
+}