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/signs_sl.cpp

@@ -26,7 +26,6 @@ static const SaveLoad _sign_desc[] = {
 	SLE_CONDVAR(Sign, owner, SLE_UINT8,                  SLV_6, SL_MAX_VERSION),
 	SLE_CONDVAR(Sign, z,     SLE_FILE_U8  | SLE_VAR_I32, SL_MIN_VERSION, SLV_164),
 	SLE_CONDVAR(Sign, z,     SLE_INT32,                SLV_164, SL_MAX_VERSION),
-	SLE_END()
 };
 
 /** Save all signs */