1.13 migration, aims & objectives

[tigerw]

I'm not entirely sure on PalettedBlockArea. The aim of this class is to have one implementation of 1.13 memory saving palettes that can be re-used in BlockArea, chunk generation, chunk storage. For the latter two though, which are comparatively short-lived but tend to be access-heavy, we'd want to prioritise speed (so no additional palette lookup) over memory savings. So the class may just end up subsumed into ChunkData for chunk storage.