Arduino: Use 400kHz clock for I²C

The EEPROM in the cartridges support 'Fast I²C' at 400kHz.
Testing it out, it seems to work fine despite the high pull-ups,
so update the Arduino code to enjoy the 4x speed-up !

Arduino/ReceiveCart/ReceiveCart.ino

@@ -45,6 +45,7 @@ void setup() {
   Serial.begin(115200);
   pinMode(LED_BUILTIN, OUTPUT);
   Wire.begin();
+  Wire.setClock(400000);
 }
 
 void loop() {

Arduino/SendCart/SendCart.ino

@@ -33,6 +33,7 @@ void setup() {
   Serial.begin(115200);
   pinMode(LED_BUILTIN, OUTPUT);
   Wire.begin();
+  Wire.setClock(400000);
 }
 
 /*