Autoregistry

Bug Fixes
* Remove circular references of `Registry` in `RegistryMeta`. This results in cleaner code, and the ability to extend the `RegistryMeta` class.

Features
* Adds a new configuration flag `hyphen=False`, which will replace underscores `_` with hyphens `-`. Can be used in conjunction with `snake_case=True` (camelCase -> snake_case conversion will be performed prior to converting underscores to hyphens).
* Adds a new configuration parameter `transform`, where a user can provide their own string-to-string transformation function for modifying registry keys.
* Renamed `registry.py` to `_registry.py` to prevent users from accidentally `from autoregistry import registry` (instead of importing `Registry`).

Bug Fixes
* More improved typing:
* add `__all__` to `__init__.py`.
* add `py.typed` marker to package.

Bug Fixes
* Internally, an element name is now strictly a `str` (previously `Optional[str]`). This can help out typing when doing reverse-key lookups. Other typing needs to be improved, but this is a common one that was easy to fix.

Breaking Change
* Subclasses of `Registry` are no longer registered to the base `Registry` class. This is to prevent cross-library mingling. Shouldn't impact most use-cases.

Bug Fixes
* `RegistryDecorator` should not be in the `Registry` registry. Fix skips registration.