FR: Iterate over rings and ranges of size N around arbitrary coordinates

These should be available as:

• const HexCoord::ring(&self, distance: usize) -> Iter<HexCoord>
• const HexCoord::range(&self, distance: usize) -> Iter<HexCoord>

We should be able to structure range() as a concatenated result of something like (0..=distance).map(|n| self.ring(n)). As for ring(), we should be able to plot out the coordinates at the corners of the ring shape, then walk between them.