Initial structure for drawer.

src/compare/diff.rs

@@ -218,6 +218,7 @@ fn compare_element<'s>(
         Element::GreaterBlock(obj) => compare_greater_block(source, emacs, obj),
         Element::FootnoteDefinition(obj) => compare_footnote_definition(source, emacs, obj),
         Element::Comment(obj) => compare_comment(source, emacs, obj),
+        Element::Drawer(obj) => todo!(),
     }
 }