Improve Debug implementation for GeometryCoordsIter::GeometryCollection

[frewsxcv]

In https://github.com/georust/geo/pull/547, we punted on implementing an insightful Debug implementation for GeometryCoordsIter::GeometryCollection. Now that GeoNum implements Debug, it might be easier for us to implement a more insightful Debug implementation.

This is the existing implementation:

https://github.com/georust/geo/blob/2c8882eb653b23b2d464ea45b4fb022a0dfb1d62/geo/src/algorithm/coords_iter.rs#L633-L636

Instead of just displaying ..., we should iterate over the items and display them.