Add world saving/loading

[Unarelith]

See #122.

[Unarelith]

Partially fixed by a355455

Note that those commands save every chunk the server has loaded so far, so don't save a world after exploring a lot, the server will hang, it'll take a lot of time to save/load the file, and it may take a lot of disk space.

They're nice if you want to save a small build near world spawn though.

Since c277dc7, save files are compressed, and since b1ed51f, only modified chunks are saved.

[Unarelith]

Missing features have their own tickets:

• Save entities (see Save entities #126)
• Save player positions and inventories (see Save player position and inventory #127)
• Save seed (see World generation should use a seed #116)
• Improve WorldSelectionState and WorldInfoState (see Improve world selection menu #128)