Serial events (#337)

* added "serial->on data received"

* updated info

* updated docs

* added readline to read entire buffer

* lazy initialize of serial buffers

* init async on event

* updated docs

docs/reference/serial.md

@@ -7,8 +7,11 @@ serial.writeLine("");
 serial.writeNumber(0);
 serial.writeValue("x", 0);
 serial.writeString("");
+serial.readUntil(",");
 serial.readLine();
+serial.readString();
 serial.redirect(SerialPin.P0, SerialPin.P0, BaudRate.BaudRate115200);
+serial.onDataReceived(",", () => {})
 ```
 
 ### See Also