Preempt further mon_id breakage and make saves use string mon_ids

* Save loaders work with monster.type.id as string or int.
* Save monster.type.id as string.
* Save kills as json.

This utilizes tile_id_data for now, however a monster jsonization is pending, and this should be a fairly easy switch:
* Anything before this commit is int, thus uses legacy_monster_id to obtain the appropriate string. (Permanent)
* After jsonization, converting string to int on load won't be required, so monster_ints is temporary.