Faster implementation of integer square root.

If you wonder how code doing multiplications can be faster than code
doing just shifts and increments: I've measured it. One million
square roots in 30 seconds with the new code instead of 220 seconds
with the old code on a Gen7 20 MHz. That's just 30 microseconds or
600 CPU cycles per root.

Code used for the measurement (by a stopwatch) in mendel.c:
...
int main (void)
{
  uint32_t i, j;

  serial_init();
  sei();
  serial_writestr_P(PSTR("start\n"));
  for (i = 0; i < 1000000; i++) {
    j = int_sqrt(i);
  }
  serial_writestr_P(PSTR("done\n"));
  delay_ms(20);
  cli();

  init();
...

--Traumflug