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/industry_sl.cpp

@@ -67,7 +67,7 @@ static const SaveLoad _industry_desc[] = {
 	SLE_CONDVAR(Industry, exclusive_supplier,         SLE_UINT8,                 SLV_GS_INDUSTRY_CONTROL, SL_MAX_VERSION),
 	SLE_CONDVAR(Industry, exclusive_consumer,         SLE_UINT8,                 SLV_GS_INDUSTRY_CONTROL, SL_MAX_VERSION),
 
-	SLEG_CONDARR(_old_ind_persistent_storage.storage, SLE_UINT32, 16,            SLV_76, SLV_161),
+	SLEG_CONDARR("storage", _old_ind_persistent_storage.storage, SLE_UINT32, 16, SLV_76, SLV_161),
 	SLE_CONDREF(Industry, psa,                        REF_STORAGE,              SLV_161, SL_MAX_VERSION),
 
 	SLE_CONDNULL(1, SLV_82, SLV_197), // random_triggers
@@ -136,7 +136,7 @@ static void Ptrs_INDY()
 
 /** Description of the data to save and load in #IndustryBuildData. */
 static const SaveLoad _industry_builder_desc[] = {
-	SLEG_VAR(_industry_builder.wanted_inds, SLE_UINT32),
+	SLEG_VAR("wanted_inds", _industry_builder.wanted_inds, SLE_UINT32),
 };
 
 /** Save industry builder. */