PHP extension prototype written in C++ implementing the new MCPE 1.2.13 subchunk format

This extension implements some performance-sensitive components of PocketMine-MP's internal chunk handling system in C++ for better performance and lower memory usage.

At the time of writing:

• \pocketmine\world\format\PalettedBlockArray: This class implements paletted block-storages as per modern MCPE since 1.2.13.
• \pocketmine\world\format\io\SubChunkConverter: This class contains a series of helper methods for upgrading legacy world terrain.