Persistence and serialization

[benpollarduk]

Allow game state to be serialised and restored.

[benpollarduk]

This needs to be done to keep alignment with NetAF.

Do in two parts.

Part 1
Make changes needed to support serialisation:

• Change to multiple players on game
• Align game creation signatures
• Add ItemContainer interface and implement on Room and PlayableCharacter
• Remove Morph from Item and fix examples
• Remove AddExit and RemoveExit from Room and fix examples
• Update readme code examples
• Update PlayableCharacter documentation to remove acquire and require methods and add add item and remove item methods
• Make Examination and Interaction private set
• Make description and commands private set
• Make conversations private set
• Add property to custom commands to set if they can be interpreted when not visible, and update interpreter and tests
• Remove Morph method from Item documentation
• Remove AddExit and RemoveExit from Room documentation
• Update documentation for examination, custom commands, conversations
• Refactor commands to include help
• Add GameMode interface and instances for all game modes
• Refactor Game.execute so that it is less complex

Part 2
Add serialisation

• Add serialise and restore from serialisation interfaces
• Add serialise for all assets, attributes and game
• Implement restore interface on all assets
• Add RestorePoint
• Add AssetArranger and associated data classes
• Add JsonFile
• Unit test all of the above
• Add example save and load commands to example game
• Add persistence documentation