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/goal_sl.cpp

@@ -21,7 +21,6 @@ static const SaveLoad _goals_desc[] = {
 	    SLE_STR(Goal, text,      SLE_STR | SLF_ALLOW_CONTROL, 0),
 	SLE_CONDSTR(Goal, progress,  SLE_STR | SLF_ALLOW_CONTROL, 0, SLV_182, SL_MAX_VERSION),
 	SLE_CONDVAR(Goal, completed, SLE_BOOL, SLV_182, SL_MAX_VERSION),
-	    SLE_END()
 };
 
 static void Save_GOAL()