Adding a peek() function to Stream and HardwareSerial (Serial).

cores/arduino/HardwareSerial.cpp

@@ -178,6 +178,15 @@ uint8_t HardwareSerial::available(void)
   return (RX_BUFFER_SIZE + _rx_buffer->head - _rx_buffer->tail) % RX_BUFFER_SIZE;
 }
 
+int HardwareSerial::peek(void)
+{
+  if (_rx_buffer->head == _rx_buffer->tail) {
+    return -1;
+  } else {
+    return _rx_buffer->buffer[_rx_buffer->tail];
+  }
+}
+
 int HardwareSerial::read(void)
 {
   // if the head isn't ahead of the tail, we don't have any characters

cores/arduino/HardwareSerial.h

@@ -49,6 +49,7 @@ class HardwareSerial : public Stream
     void begin(long);
     void end();
     virtual uint8_t available(void);
+    virtual int peek(void);
     virtual int read(void);
     virtual void flush(void);
     virtual void write(uint8_t);

cores/arduino/Stream.h

@@ -27,6 +27,7 @@ class Stream : public Print
 {
   public:
     virtual uint8_t available() = 0;
+    virtual int peek() = 0;
     virtual int read() = 0;
     virtual void flush() = 0;
 };