Serial: add bind() API for transparent passthrough

cores/arduino/UART.h

@@ -168,11 +168,14 @@ class UartClass : public HardwareSerial
     using Print::write; // pull in write(str) and write(buf, size) from Print
     explicit operator bool() { return true; }
 
+    void bind(UartClass& ser) {bound = &ser; }
+
     // Interrupt handlers - Not intended to be called externally
     inline void _rx_complete_irq(void);
     void _tx_data_empty_irq(void);
   private:
     void _poll_tx_data_empty(void);
+    UartClass* bound = NULL;
 };
 
 #if defined(HWSERIAL0)

cores/arduino/UART_private.h

@@ -66,6 +66,9 @@ void UartClass::_rx_complete_irq(void)
       _rx_buffer[_rx_buffer_head] = c;
       _rx_buffer_head = i;
     }
+    if (bound != NULL) {
+      bound->write(c);
+    }
   } else {
     // Parity error, read byte but discard it
     (*_hwserial_module).RXDATAL;