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/types/lesser_element.rs

@@ -149,6 +149,7 @@ pub struct BabelCall<'s> {
 pub struct LatexEnvironment<'s> {
     pub source: &'s str,
     pub name: Option<&'s str>,
+    pub value: &'s str,
 }
 
 /// A line number used in switches to lesser blocks.