Serialization merge
Following up #27, @generalmimon done tremendous job at implementing serialzation for some languages. However, it's not yet merged into `master`, thus so far these live into separate "serialization" branches:

* https://github.com/kaitai-io/kaitai_struct_compiler/tree/serialization
* https://github.com/kaitai-io/kaitai_struct_tests/tree/serialization
* https://github.com/kaitai-io/kaitai_struct_java_runtime/tree/serialization
* https://github.com/kaitai-io/kaitai_struct_python_runtime/tree/serialization

It's documented in https://doc.kaitai.io/serialization.html, but ultimately our goal obviously would be not supporting two separate forks, but having one that does everything.

I wanted to start making itemized list of tasks of what exactly needs to be done to merge these back to master.

@generalmimon, can I ask you to share your thoughts on this?