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/autoreplace_sl.cpp

@@ -21,7 +21,6 @@ static const SaveLoad _engine_renew_desc[] = {
 	    SLE_REF(EngineRenew, next,     REF_ENGINE_RENEWS),
 	SLE_CONDVAR(EngineRenew, group_id, SLE_UINT16, SLV_60, SL_MAX_VERSION),
 	SLE_CONDVAR(EngineRenew, replace_when_old, SLE_BOOL, SLV_175, SL_MAX_VERSION),
-	SLE_END()
 };
 
 static void Save_ERNW()