Rewrite the parsers in Kaitai Struct

[KOLANICH]

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Description of issue or feature request:
Currently handcoded parsers are used. It is not a very flexible approach. It complicates the matters a lot, both understanding and modification.

There exists a DSL called Kaitai Struct for describing binary parsers. It allows to describe the structure in a declarative way and then compile it into a parser in multiple programming languages.

It may make sense to replace handcoded parsers with generated ones.

BTW, https://github.com/kaitai-io/kaitai_struct_formats/blob/master/security/openpgp_message.ksy may be helpful.

[joshuagl]

Thanks for the suggestion, this is the first time I've heard of Kaitai.