phase out the level 2/3 serialization in `snstype.Message`

[hupili]

Those are pre-mature features and time has proved that only level 1 serialization is widely used.

[hupili]

more notes:

• dump_parsed is useful but it is very easy to access. Make the parsed field pretty-print by default in next upgrade.
• dump_full was considered useful at first because it contains all the information. However, we do not have mature use case from other languages. We won't have it for a long time. As for Python, there are more better methods. e.g. utils.obj2str which has real use case.