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/script/script_instance.cpp

@@ -345,7 +345,7 @@ static byte _script_sl_byte; ///< Used as source/target by the script saveload c
 
 /** SaveLoad array that saves/loads exactly one byte. */
 static const SaveLoad _script_byte[] = {
-	SLEG_VAR(_script_sl_byte, SLE_UINT8),
+	SLEG_VAR("type", _script_sl_byte, SLE_UINT8),
 };
 
 /* static */ bool ScriptInstance::SaveObject(HSQUIRRELVM vm, SQInteger index, int max_depth, bool test)