SymbolGenerator: add chr() and made constexpr-capable

[jlblancoc]

That it is:

• add a chr() method to recover the underlying char symbol, and
• Make possible to write things like:

constexpr auto sQpp = gtsam::SymbolGenerator('a');
//...

    for (const auto &kv : values) {
      gtsam::Symbol s(kv.key);
      switch (s.chr()) {
      case sQpp.chr():    // this requires SymbolGenerator to be constexpr
        // ...
      }
   }

[varunagrawal]

LGTM