FR: Rotate coordinates around other arbitrary coordinates

This should be available as:

• const HexCoord::rotate_around_cw(&self, other: HexCoord, n: usize) -> HexCoord
• const HexCoord::rotate_around_ccw(&self, other: HexCoord, n: usize) -> HexCoord
• const HexCoord::rotate_cw(&self, n: usize) -> HexCoord
• const HexCoord::rotate_ccw(&self, n: usize) -> HexCoord

Rotation around the origin is pretty easy, and just involves transforming the axes of the coordinate being rotated (once per rotation):

CW:  (q, r, s) -> (-r, -s, -q)
CCW: (q, r, s) -> (-s, -q, -r)

For arbitrary origins (not just (0, 0, 0)) we can center the coordinate before rotation and re-offset it afterwards:

impl HexCoord {
  // ...

  const fn rotate_around_cw(&self, other: HexCoord, n: usize) -> HexCoord {
    (self - other).rotate_cw(n) + other
  }
}