Bug TW-1254

- TW-1254 Calc command can segfault on negative numbers (thanks to Renato
          Alves).
- Implemented a recursive descent parser to syntax check the expression,
  and disambiguate unary minus. The syntax checking is not generating any
  diagnostics yet.

src/ISO8601.h

@@ -59,6 +59,7 @@ private:
   int dayOfWeek (int, int, int);
   bool validate ();
   void resolve ();
+  void dump ();
 
 public:
   bool _ambiguity;