Enable changing the serial device.

[mike-matera]

This patch is tested on Uno using the default and a SoftwareSerial instance. Changing the serial port on Leonardo compiles but I is not tested.

This fixes bug #20

Would someone please help by testing their use case?

Here's an example that uses SoftwareSerial

#include <ArduinoSTL.h>
#include <SoftwareSerial.h>

SoftwareSerial mySerial(0, 1); // RX, TX (Hardware pins on Uno)

namespace std { 
  ohserialstream cout(mySerial);
  ihserialstream cin(mySerial);
}

void setup() {
  mySerial.begin(9600);
  ArduinoSTL_Serial.connect(mySerial);
}

void loop() {
  printf ("Hello ");
  std::cout << "World!" << std::endl;
}

[neu-rah]

works ok on nano (atmega328) and pro-mini (atmega32u4), it messes, as expected, with other boards like esp32, eps8266, nrf52, etc.. even if they support arduino framework and because stl is already available on those platforms.
keep up the good work 👍

[mike-matera]

Thanks! I'm going to merge this change.