Improve scenario and faction generation

[bensku]

• Explicitly generate backstory for the ruler and edit kingdom description to match it
• Parallelize world generation
• Retry when encountering LLM timeouts
• Implement saving and loading worlds

Known bugs:

• test_worldgen.py may hang after everything has been generated
• Only gpt-3.5-chat generates good results, for some reason

Example world, converted to markdown with dump_world.py: https://gist.github.com/bensku/32eaa10149dfe4794eb52bc2bb51b381