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/object_sl.cpp

@@ -25,8 +25,6 @@ static const SaveLoad _object_desc[] = {
 	SLE_CONDVAR(Object, colour,                     SLE_UINT8,                  SLV_148, SL_MAX_VERSION),
 	SLE_CONDVAR(Object, view,                       SLE_UINT8,                  SLV_155, SL_MAX_VERSION),
 	SLE_CONDVAR(Object, type,                       SLE_UINT16,                 SLV_186, SL_MAX_VERSION),
-
-	SLE_END()
 };
 
 static void Save_OBJS()