Implement name reader

[vitaut]

Some solvers or solver-like programs such as smpswriter require access to variable and constraint names. The name reader should be separate from NLReader for users to be able to provide names and NL input from different sources. Possible API:

class NameHandler {
 public:
  void OnVarName(fmt::StringRef name);
  void OnConName(fmt::StringRef name);
};

template <typename NameHandler>
void ReadNameFiles(fmt::StringRef row_filename, fmt::StringRef col_filename,
                   NameHandler &handler);