FR: Get the number of steps necessary to get from one coord to another

This should be available as HexCoord::steps_to(&self, other: HexCoord) -> usize. In cubic coordinates, we can find the distance like so:

q' = self.q - other.q
r' = self.r - other.r
s' = self.s - other.s

d = (|q'| + |r'| + |s'|) / 2

With this, we should be able to get distance from the origin with something along the lines of:

impl HexCoord {
  // ...
  
  const fn length(&self) -> usize {
    self.steps_to(HexCoord::ZERO)
  }
}