Miniagents

- Singleton and ModelSingleton in utils.py

- new example: self_dev
- README.md - preliminary draft (still contains inaccurate information)
- users can enter multiline messages in "conversation" example

- Rename `Node` to `Frozen`
- Introduce `MessagePromise.preliminary_metadata` for fields that are available even before tokens are collected

- Improved naming of things (names in Promising made more intuitive)
- Improved visibility of exceptions

- openai support
- utils.achain_loop
- utils.split_messages

MiniAgents.on_serialize_message