Implement saving players to and loading players from a save file

Blocked by #17
* Extend the existing save functionality to also save players to the same save file as monsters.
* Create player variants of parsing the players from a save file.

Start by reading the blocked issue #17 and tracing the existing monster save and load functionality. Extend that path so players share the same save file, with player-specific parsing; done means players persist and reload alongside monsters.

Written by the indexing model from the issue text.