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/depot_sl.cpp

@@ -25,7 +25,6 @@ static const SaveLoad _depot_desc[] = {
 	 SLE_CONDVAR(Depot, town_cn,    SLE_UINT16,               SLV_141, SL_MAX_VERSION),
 	SLE_CONDSSTR(Depot, name,       SLE_STR,                  SLV_141, SL_MAX_VERSION),
 	 SLE_CONDVAR(Depot, build_date, SLE_INT32,                SLV_142, SL_MAX_VERSION),
-	 SLE_END()
 };
 
 static void Save_DEPT()