In HardwareSerial::write, bypass the queue when it's empty

This helps improve the effective datarate on high (>500kbit/s) bitrates,
by skipping the interrupt and associated overhead. At 1 Mbit/s the
implementation previously got up to about 600-700 kbit/s, but now it
actually gets up to the 1Mbit/s (values are rough estimates, though).

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

@@ -197,6 +197,15 @@ void HardwareSerial::flush()
 
 size_t HardwareSerial::write(uint8_t c)
 {
+  // If the buffer and the data register is empty, just write the byte
+  // to the data register and be done. This shortcut helps
+  // significantly improve the effective datarate at high (>
+  // 500kbit/s) bitrates, where interrupt overhead becomes a slowdown.
+  if (_tx_buffer_head == _tx_buffer_tail && bit_is_set(*_ucsra, UDRE0)) {
+    *_udr = c;
+    sbi(*_ucsra, TXC0);
+    return 1;
+  }
   int i = (_tx_buffer_head + 1) % SERIAL_BUFFER_SIZE;
 
   // If the output buffer is full, there's nothing for it other than to