Make the item tag exit matcher a lower class than all all others.

This is to allow for " :: " inside a description list item's tag if it is nested inside another object.

src/context/exiting.rs

@@ -1,16 +1,10 @@
 #[derive(Debug, Copy, Clone)]
 pub enum ExitClass {
-    /// Headlines and sections.
     Document = 1,
-
-    /// Elements who take priority over beta elements when matching.
-    Alpha = 20,
-
-    /// Elements who cede priority to alpha elements when matching.
-    Beta = 300,
-
-    /// Elements who cede priority to alpha and beta elements when matching.
-    Gamma = 4000,
+    Alpha = 2,
+    Beta = 3,
+    Gamma = 4,
+    Delta = 5,
 }
 
 impl std::fmt::Display for ExitClass {