Add a main menu scene with Start/Hard Mode/Music options

The game now boots into a menu (Start Game, Hard Mode toggle, Music
toggle) navigated with Up/Down and A/Start, instead of dropping
straight into a match. B returns to the menu from a paused or
game-over state.

include/game_state.h

@@ -6,6 +6,13 @@
 #include <vector>
 #include <nds/ndstypes.h>
 
+enum class Scene {
+    Menu,
+    Playing,
+};
+
+constexpr int MENU_ITEM_COUNT = 3;
+
 struct GameState {
     volatile uint16_t ticks = 0;
     volatile uint16_t snakelength = 0;
@@ -16,6 +23,9 @@ struct GameState {
     bool paused = false;
     bool hardMode = false;
 
+    Scene scene = Scene::Menu;
+    int menuSelection = 0;
+
     uint16_t curseTicksRemaining = 0;
 
     std::deque<std::array<int, 3>> snake;