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/newgrf_sl.cpp

@@ -20,7 +20,6 @@ static const SaveLoad _newgrf_mapping_desc[] = {
 	SLE_VAR(EntityIDMapping, grfid,         SLE_UINT32),
 	SLE_VAR(EntityIDMapping, entity_id,     SLE_UINT8),
 	SLE_VAR(EntityIDMapping, substitute_id, SLE_UINT8),
-	SLE_END()
 };
 
 /**
@@ -65,7 +64,6 @@ static const SaveLoad _grfconfig_desc[] = {
 	    SLE_ARR(GRFConfig, param,            SLE_UINT32, 0x80),
 	    SLE_VAR(GRFConfig, num_params,       SLE_UINT8),
 	SLE_CONDVAR(GRFConfig, palette,          SLE_UINT8,  SLV_101, SL_MAX_VERSION),
-	SLE_END()
 };