Avoid copying cached RegExes for scalars

This improves performance on the test.yaml attached to #158 by about
25% on my machine as compared to the previous commit (0.25s -> 0.20s),
as measured by `time build/util/parse < test.yaml > /dev/null`.

src/exp.h

@@ -20,6 +20,10 @@ namespace YAML {
 
 namespace Exp {
 // misc
+inline const RegEx& Empty() {
+  static const RegEx e;
+  return e;
+}
 inline const RegEx& Space() {
   static const RegEx e = RegEx(' ');
   return e;