Refactored out the main_loop so that it's not tightly coupled inside builder, and in the process found I was accidentally copying GUI and GameEngine because Builder wasn't using a &ref for them. Now they don't have a copy constructor to catch that.

escape_turings_tarpit.cpp

@@ -1,4 +1,5 @@
 #include "builder.hpp"
+#include "gui.hpp"
 #include <fmt/core.h>
 
 int main(int argc, char *argv[])
@@ -7,7 +8,7 @@ int main(int argc, char *argv[])
     GameEngine game{100};
 
     auto builder = Builder(gui, game);
-    builder.run();
+    gui.main_loop(game, builder);
 
     return 1;
 }