&meeseengine Voxel Engine for Dreamcast

Added a new inventory system along with support for destructive block drops (grass -> dirt, stone -> cobblestone, leaves -> nothing). To keep memory usage under control, since each inventory is hundreds of bytes, they're immutable and automatically de-duplicated. Most entities share the same items (like cows dropping beef and leather), so interning avoids unnecessary duplication and also sets the stage for efficient network rollback in multiplayer.