Improve error messages in "robot-simulator"

Start with robot_simulator_test.cpp and the existing Catch2 test setup. Review Catch2's StringMaker and CATCH_REGISTER_ENUM documentation, then add the proposed diagnostic support and run the robot-simulator tests; done means failed comparisons show pair values and Bearing names instead of placeholders or integers.

The tests in the exercise "robot-simulator" compare the result of the member functions get_position() and get_bearing() with the operator== like this:

TEST_CASE("A_robots_is_created_with_a_position_and_a_direction")
{
    const Robot r;

    const std::pair<int, int> expected_robot_position{0, 0};
    REQUIRE(expected_robot_position == r.get_position());
    REQUIRE(Bearing::NORTH == r.get_bearing());
}

But Catch2 does not know how to print a std::pair, and it prints an enum or enum class like an integer:

-------------------------------------------------------------------------------
A_robots_is_created_with_a_position_and_a_direction
-------------------------------------------------------------------------------
/home/user/exercism/cpp/robot-simulator/robot_simulator_test.cpp:13
...............................................................................

/home/user/exercism/cpp/robot-simulator/robot_simulator_test.cpp:18: FAILED:
  REQUIRE( expected_robot_position == r.get_position() )
with expansion:
  {?} == {?}


-------------------------------------------------------------------------------
A_robots_is_created_with_a_position_and_a_direction
-------------------------------------------------------------------------------
/home/user/exercism/cpp/robot-simulator/robot_simulator_test.cpp:13
...............................................................................

/home/user/exercism/cpp/robot-simulator/robot_simulator_test.cpp:19: FAILED:
  REQUIRE( Bearing::NORTH == r.get_bearing() )
with expansion:
  0 == 2

That's not really helpful.

Catch2 has the a StringMaker for printing custom classes (see the documentation), and it has the macro CATCH_REGISTER_ENUM() for better error messages when working with enums (see the documentation).

By adding a few lines somewhere at the beginning of robot_simulator_test.cpp

// for better error messages
namespace Catch
{
    template <typename T1, typename T2>
    struct StringMaker<std::pair<T1, T2>>
    {
        static std::string convert(const std::pair<T1, T2>& value)
        {
            std::string result = "std::pair{";
            result += StringMaker<T1>::convert(value.first);
            result += ", ";
            result += StringMaker<T2>::convert(value.second);
            result += '}';
            return result;
        }
    };
}
CATCH_REGISTER_ENUM(robot_simulator::Bearing,
        robot_simulator::Bearing::NORTH,
        robot_simulator::Bearing::WEST,
        robot_simulator::Bearing::SOUTH,
        robot_simulator::Bearing::EAST)

we would get better error messages:

-------------------------------------------------------------------------------
A_robots_is_created_with_a_position_and_a_direction
-------------------------------------------------------------------------------
/home/user/exercism/cpp/robot-simulator/robot_simulator_test.cpp:36
...............................................................................

/home/user/exercism/cpp/robot-simulator/robot_simulator_test.cpp:41: FAILED:
  REQUIRE( expected_robot_position == r.get_position() )
with expansion:
  std::pair{0, 0} == std::pair{0, 1}


-------------------------------------------------------------------------------
A_robots_is_created_with_a_position_and_a_direction
-------------------------------------------------------------------------------
/home/user/exercism/cpp/robot-simulator/robot_simulator_test.cpp:36
...............................................................................

/home/user/exercism/cpp/robot-simulator/robot_simulator_test.cpp:42: FAILED:
  REQUIRE( Bearing::NORTH == r.get_bearing() )
with expansion:
  NORTH == SOUTH

AFAIK there are only two possible problems:

1. robot_simulator_test.cpp becomes more complex. But IMHO those 22 lines can be ignored.

2. That would effectively enforce the use of enum or enum class for Bearing. IMHO that's not a problem for us because we want idiomatic solutions, and that's enum or better enum class.

IMHO the benefits outweigh these problems.
What do you folks think?