Replace_startMillis from Stream.h with local variables

The member _startMillis in the class Stream is unneccesarry and can be replace with a local variable within the methods timedRead() and timedPeek(). This saves 4bytes per Stream instance...

    timedPeek() {
       unsigned long start = millis();
       int c = peek();
        while(c < 0 && millis() - start < timout) {
          c = peek();
        }
        return c; 
     }

I guess, the implemenation above should produce less code then the original one with the do-while-loop. At least, i find it more readable.

Also, timedRead() could be implemented using timedPeek()

    timedRead() {
        return timedPeek() < 0 ? -1 : read();
    }

This again, should produce less code - At least, it removes doublicated code...