inventory: purge unused methods, fix invcache load
player: carve off old save stringstream loader into load_legacy
player/npc/related classes; split off old loading into *::load_legacy,
pure old stringstream saves
monster/item: split off old loading into *::load_legacy
vehicle: split off legacy loader
move all load_legacy class functions to savegame_legacy.cpp
monster/item/inventory: move json_save/load functions to savegame_json.cpp to join the rest