Add a test showing a text markup condition we are not handling and significantly reduce allocations by using references for the captured marker for text markup.

org_mode_samples/object/text_markup/double_star.org

@@ -0,0 +1 @@
+foo ** bar ** baz

src/parser/text_markup.rs

@@ -163,8 +163,7 @@ fn text_markup_object<'c>(
     OrgSource<'s>,
 ) -> Res<OrgSource<'s>, Vec<Object<'s>>>
        + 'c {
-    let marker_symbol = marker_symbol.to_owned();
-    move |context, input: OrgSource<'_>| _text_markup_object(context, input, marker_symbol.as_str())
+    move |context, input: OrgSource<'_>| _text_markup_object(context, input, marker_symbol)
 }
 
 #[cfg_attr(feature = "tracing", tracing::instrument(ret, level = "debug"))]