Use a specialized null value for ChunkPos

This prevents logic errors related to being in chunk 0 0 0.

Also:

* Moved some duplicated chunk teleportation logic to a new helper
  function
* Made ChunkPos into a proper class so it can default to INVALID_CHUNK
  when default-initialized
* Reversed the inclusion order of Chunking.hpp and Entities.hpp to work
  around problems with type definitions

src/Entities.hpp

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "core/Core.hpp"
+#include "Chunking.hpp"
 
 #include <stdint.h>
 #include <set>
@@ -15,8 +16,6 @@ enum class EntityType : uint8_t {
     BUS
 };
 
-class Chunk;
-
 struct Entity {
     EntityType type = EntityType::INVALID;
     int x = 0, y = 0, z = 0;