Move consuming trailing element whitespace inside the parsers.

This ensures the parsers can take into account the affiliated keywords when setting their source without needing the SetSource trait.

src/compare/diff.rs

@@ -2497,11 +2497,11 @@ fn compare_latex_environment<'b, 's>(
 
     // Compare value
     let value = get_property_quoted_string(emacs, ":value")?;
-    if value.as_ref().map(String::as_str) != Some(rust.source) {
+    if value.as_ref().map(String::as_str) != Some(rust.value) {
         this_status = DiffStatus::Bad;
         message = Some(format!(
             "Value mismatch (emacs != rust) {:?} != {:?}",
-            value, rust.source
+            value, rust.value
         ));
     }