Cosmic-toolkit

Added
- **Breaking change!** Added `AggregateRoot` that aggregate roots need to inherit from,
in addition to Entity; moved domain event logic to AggregateRoot
- `types` module for commonly used types

Changed
- **Breaking change!** `AbstractRepository`'s entity_type class-level keyword argument
now expects the entity to be a subclass of AggregateRoot. Each aggregate root should
have a repository and entities are children of the root entity.
- `BaseUnitOfWork.collect_new_events()` now pops events from `AggregateRoot.events` to
ensure that no event is double-published

Fixed
- Incorrectly instantiated entities in a few tests. Entities in `conftest.py` now
conform to the `AggregateRoot` interface.

Changed
- Updated AbstractRepository to save class-level keyword arguments so they can be used
by subclasses if needed
- Moved repository instantiation in BaseUnitOfWork from __init__() to __aenter__() to
allow subclasses to manipulate arguments that are passed into repositories

Added
- add_dependencies() method to MessageBus class
- __repr__() method to AbstractRepository, BaseUnitOfWork, and Entity