Asphalt-serialization

- Added the ability to skip wrapping custom marshalled objects (by setting ``wrap_state=False`` in
any of the customizable serializers)

- Allow parameterless unmarshaller callbacks that return a new instance of the target class
- Switched YAML serializer to use ruamel.yaml instead of PyYAML

- **BACKWARD INCOMPATIBLE** Switched the CBOR implementation to
`cbor2 <https://pypi.io/project/cbor2/>`_
- **BACKWARD INCOMPATIBLE** Switched msgpack's MIME type to ``application/msgpack``
- **BACKWARD INCOMPATIBLE** Switched the default context attribute name to the backend name,
for consistency with asphalt-templating
- Added custom type handling for CBOR, msgpack and JSON serializers
- Serializer resources are now also published using their actual types (in addition the interfaces)

- **BACKWARD INCOMPATIBLE** Migrated to Asphalt 2.0
- **BACKWARD INCOMPATIBLE** A backend must be specified explicitly (it no longer defaults to JSON)
- Allowed combining ``serializers`` with default parameters

- Added support for CBOR (Concise Binary Object Representation)
- Added typeguard checks to fail early if arguments of wrong types are passed to functions

- Initial release