Cbor2

- Fixed decoding failure for datetimes with microseconds (tag 0)

- Fixed error in the cyclic structure detection code that could mistake one container for
another, sometimes causing a bogus error about cyclic data structures where there was none

- **BACKWARD INCOMPATIBLE** Encoder callbacks now receive three arguments: the encoder instance,
the value to encode and a file-like object. The callback must must now either write directly to
the file-like object or call another encoder callback instead of returning an iterable.
- **BACKWARD INCOMPATIBLE** Semantic decoder callbacks now receive four arguments: the decoder
instance, the primitive value, a file-like object and the shareable index for the decoded value.
Decoders that support value sharing must now set the raw value at the given index in
``decoder.shareables``.
- **BACKWARD INCOMPATIBLE** Removed support for iterative encoding (``CBOREncoder.encode()`` is no
longer a generator function and always returns ``None``)
- Significantly improved performance (encoder ~30 % faster, decoder ~60 % faster)
- Fixed serialization round-trip for ``undefined`` (simple type 23)
- Added proper support for value sharing in callbacks

- **BACKWARD INCOMPATIBLE** Deserialize unknown tags as ``CBORTag`` objects so as not to lose
information
- Fixed error messages coming from nested structures

- Fixed deserialization of cyclic structures

- Initial release