Move objects to a lower exit class.

Paragraph's exit matcher which detects elements was causing the plain list parser to exit after the first item was parsed which was causing significant amounts of re-parsing.

src/parser/paragraph.rs

@@ -28,7 +28,7 @@ pub fn paragraph<'r, 's>(
 ) -> Res<OrgSource<'s>, Paragraph<'s>> {
     let parser_context =
         context.with_additional_node(ContextElement::ExitMatcherNode(ExitMatcherNode {
-            class: ExitClass::Beta,
+            class: ExitClass::Gamma,
             exit_matcher: &paragraph_end,
         }));
     let standard_set_object_matcher = parser_with_context!(standard_set_object)(&parser_context);