Use uint8_t for HardwareSerial ringbuffer pointers

Since the buffers aren't bigger than 64 bytes, these values can be
smaller. This saves a few bytes of ram, but also saves around 50 bytes
of program space, since the values can now be loaded using a single
instruction.

To prevent problems when people manually increase the buffer size, a
compile-time check is added.

Closes: arduino#1078

Author: matthijskooijman

hardware/arduino/avr/cores/arduino/HardwareSerial.cpp

@@ -62,10 +62,14 @@
 struct ring_buffer
 {
   unsigned char buffer[SERIAL_BUFFER_SIZE];
-  volatile unsigned int head;
-  volatile unsigned int tail;
+  volatile uint8_t head;
+  volatile uint8_t tail;
 };
 
+#if SERIAL_BUFFER_SIZE > 256
+#error Serial buffer size too big for head and tail pointers
+#endif
+
 #if defined(USBCON)
   ring_buffer rx_buffer = { { 0 }, 0, 0};
   ring_buffer tx_buffer = { { 0 }, 0, 0};