Optimize Polygon subdivision

Increases performance and reduces memory allocations. The times in the table below are in ms.

| Polygons | master | This branch |
| ------------ | --------- | --------------- |
| US states | 59.7 | 43.4 |
| US counties | 930.4 | 257.4 |
| Countries | 10747.2 | 5620.7 |
Do you think we would get a performance gain by optimizing this key to avoid the string, e.g., a floating-point value that is `min.max` or a more efficient data structure than just using a JavaScript object as a hash lookup?