c++ wrapper around the c API for gprolog.

Build and run example with:

      > make example
      > ./example

Example use of wrapper API:

      query q1;

      q1.add( 
        call { []() -> void {
          const auto nil_at = mk_atom( nil {} );
          cout << "try write nil" << endl;
          int res = oneSoln {"write",{ mk_term(nil_at)}} (true);
        }} 
      ).add(
        call { []() -> void {
          int res;
          const auto nil_at = mk_atom( nil {} );
          oneSoln mycall("length", {
            mk_term(nil_at), mk_term(variable{}) 
          });
          res = mycall(true);

          oneSoln writeArg("write", { mycall.arg(1) } ); 
          res = writeArg(true);

        }}
      );

      program p {};
      p.add(q1);
      p();