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We won't be able to make it fully self-descriptive (looking at you MAP-chunks), but anything else can. With this framework, we can add headers for each chunk explaining how each chunk looks like in detail. They also will all be tables, making it a lot easier to read in external tooling, and opening the way to consider a database (like SQLite) to use as savegame format. Lastly, with the headers in the savegame, you can freely add fields without needing a savegame version bump; older versions of OpenTTD will simply ignore the new field. This also means we can remove all the SLE_CONDNULL, as they are irrelevant. The next few commits will start using this framework.