Feature: framework to make savegames self-descriptive

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.

src/saveload/cheat_sl.cpp

@@ -74,7 +74,6 @@ static void Load_CHTS()
 	if (!IsSavegameVersionBefore(SLV_RIFF_TO_ARRAY) && SlIterateArray() != -1) SlErrorCorrupt("Too many CHTS entries");
 }
 
-/** Chunk handlers related to cheats. */
 static const ChunkHandler cheat_chunk_handlers[] = {
 	{ 'CHTS', Save_CHTS, Load_CHTS, nullptr, nullptr, CH_ARRAY },
 };