v2.0.0

• ⚠️ 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.