factor out simple templates into a separate SimpleTemplateDir

there's no need to bloat TreehouseDir with that logic

src/vfs.rs

@@ -168,6 +168,12 @@ impl<'a> dyn Erased<'a> + 'a {
     }
 }
 
+impl Dir for () {
+    fn query(&self, _path: &VPath, _query: &mut Query) {
+        // Noop implementation.
+    }
+}
+
 impl<T> Dir for &T
 where
     T: Dir,