Codechange: C++-ify lists for SaveLoad (#9323)

Basically, this changes "SaveLoad *" to either:
1) "SaveLoadTable" if a list of SaveLoads was meant
2) "SaveLoad &" if a single entry was meant

As added bonus, this removes SL_END / SLE_END / SLEG_END. This
also adds core/span.hpp, a "std::span"-lite.

src/saveload/engine_sl.cpp

@@ -42,8 +42,6 @@ static const SaveLoad _engine_desc[] = {
 	SLE_CONDSSTR(Engine, name,                SLE_STR,                    SLV_84, SL_MAX_VERSION),
 
 	SLE_CONDNULL(16,                                                       SLV_2, SLV_144), // old reserved space
-
-	SLE_END()
 };
 
 static std::vector<Engine*> _temp_engine;
@@ -173,7 +171,6 @@ static const SaveLoad _engine_id_mapping_desc[] = {
 	SLE_VAR(EngineIDMapping, internal_id,   SLE_UINT16),
 	SLE_VAR(EngineIDMapping, type,          SLE_UINT8),
 	SLE_VAR(EngineIDMapping, substitute_id, SLE_UINT8),
-	SLE_END()
 };
 
 static void Save_EIDS()