World loading/saving and major refactoring

See https://github.com/MrSmith33/voxelman/issues?q=milestone%3Av0.3.0
Video

More features and more bugs

After moving and loading new chunks and unloading old ones something happens and application hangs or Invalid Memory Operation appears in the console.
Disabling GC helps.
Not deleting chunks also helps.
Seems like error happens in another thread where chunk meshes are generated.

v0.1.0