xo-reader2 stack: expand symbol table to store typedefs

+ typedef utest
+ misc qol policy choices

include/xo/expression2/DVariable.hpp

@@ -76,7 +76,7 @@ namespace xo {
 
         private:
             /** symbol name **/
-            const DUniqueString * name_;
+            const DUniqueString * name_ = nullptr;
             /** variable value always has type consistent
              *  with this description
              **/