Add an optimization idea.

notes/optimization_ideas.org

@@ -25,3 +25,4 @@ This could significantly reduce our calls to exit matchers.
 I think targets would break this.
 
 The exit matchers are already implicitly building this behavior since they should all exit very early when the starting character is wrong. Putting this logic in a centralized place, far away from where those characters are actually going to be used, is unfortunate for readability.
+** Use exit matcher to cut off trailing whitespace instead of re-matching in plain lists.