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/subsidy_sl.cpp

@@ -24,7 +24,6 @@ static const SaveLoad _subsidies_desc[] = {
 	SLE_CONDVAR(Subsidy, src,        SLE_UINT16,                  SLV_5, SL_MAX_VERSION),
 	SLE_CONDVAR(Subsidy, dst,        SLE_FILE_U8 | SLE_VAR_U16,   SL_MIN_VERSION, SLV_5),
 	SLE_CONDVAR(Subsidy, dst,        SLE_UINT16,                  SLV_5, SL_MAX_VERSION),
-	SLE_END()
 };
 
 static void Save_SUBS()