Replaced the START_ROBOT_CLASS() macro with a template function (#1050)

The START_ROBOT_CLASS() macro's main() now calls this function through a
deprecated proxy function to encourage users to switch.

wpilibcExamples/src/main/cpp/examples/GearsBot/cpp/Robot.cpp

@@ -42,4 +42,4 @@ void Robot::TeleopPeriodic() { frc::Scheduler::GetInstance()->Run(); }
 
 void Robot::TestPeriodic() {}
 
-START_ROBOT_CLASS(Robot)
+int main() { return frc::StartRobot<Robot>(); }