Flatbush

Fix infinite loop on `new Flatbush(0)` (throws an error now).

Fixed `search` not throwing an error if called before `finish`.

- Slightly improved search performance.

- Improved search performance by ~30%.
- Slightly better packing for cases where item bbox sizes vary a lot.

- ⚠️ Breaking: `flatbush(...)` should now be `new Flatbush(...)`.
- Add optional `data` constructor argument for reconstructing an index after transferring.
- Use ES syntax in the exported module (doesn't affect node/browser builds).
- Expose `numItems`, `nodeSize`, `ArrayType`, `minX`, `minY`, `maxX`, `maxY` properties.
- Avoid worst case performance when data is already hilbert-sorted.

Fixed a sorting bug, in turn significantly improving search performance.