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/group_sl.cpp

@@ -26,7 +26,6 @@ static const SaveLoad _group_desc[] = {
 	 SLE_CONDVAR(Group, livery.colour1,     SLE_UINT8,                     SLV_GROUP_LIVERIES, SL_MAX_VERSION),
 	 SLE_CONDVAR(Group, livery.colour2,     SLE_UINT8,                     SLV_GROUP_LIVERIES, SL_MAX_VERSION),
 	 SLE_CONDVAR(Group, parent,             SLE_UINT16,                    SLV_189, SL_MAX_VERSION),
-	     SLE_END()
 };
 
 static void Save_GRPS()