Support Timestamp extension type

[jm-]

It would be great if the timestamp extension type is supported out of the box.

It seems like there are already some functions that test DateTime deserialization, but these aren't wired up to the main test entrypoint. I have tried running these locally and the serialized DateTime currently deserializes to <null>.

$ dart test
Building package executable...
Built test:test.
00:00 +0 -1: test\messagepack_test.dart: unpackSmallDateTime [E]
  Expected: DateTime:<1970-01-01 13:00:00.000>
    Actual: <null>

  package:test_api                 expect
  test\messagepack_test.dart 14:3  unpackSmallDateTime

[AntonEriksson978]

Yes, I also have this problem. This msgpack implementation doesn't seem to be able to parse datetime.